The Croft Acoustics QUAD II KT88 Amplifiers - Review 2011

The Croft-modified QUAD II Monoblock Amplifiers combine classic British valve heritage with exceptional bespoke engineering modifications by the late Glenn Croft for transcendent audio performance.

Discover the classic QUAD II Monoblock Amplifiers – upgraded by the late Glenn Croft

T hese Croft-modified QUAD II monoblocks sit at the heart of my system, driving the Tannoy Monitor Gold loudspeakers with grace and authority. It's a classic combination—British valve heritage meeting British loudspeaker mastery. The synergy is unmistakable.

After using many transistorised amplifiers, I wanted to create a real vintage, circa 1960s system, so with that itch to scratch, a pair of QUAD II's were a perfect match together with my Tannoy Monitor Gold loudspeakers in Lancaster cabinets and my Thorens TD124 MKII turntable.

But before we get to the QUAD II's, in my search for a perfect set of vintage monoblock amplifiers I first bought a pair of LEAK TL12+ monos together with a LEAK Point One preamplifier. The Point One was actually converted to a passive unit and using it with several amplifiers afterwards, it sounded really nice indeed.

The TL12+ monoblocks looked smashing, but I found that they just didn't seem to drive the Tannoys as well as I wished.

On low volume with uncomplicated music, they sounded really lush but try to fire them up with some dance or heavy rock and, well, forget it. The sound stage simply fell apart at the seams.

I was a little surprised by this as the Tannoys are really easy to drive but unfortunately, it wasn't to be.

So, in steps the QUAD II's.

These beautiful QUAD II monoblocks were converted from using KT66's to KT88's by Glenn Croft. Unfortunately, when they were converted was unknown to me, but Glenn Croft was nicely surprised to get them back for a service, so my guess was early to mid‑1990s.

When I bought them, they were using the Gold Lion KT88 valves accompanied with Brimar and Mullard signal and regulator valves. The casework was at some point re‑sprayed to a brighter grey/blue colour which updated their look nicely.

When they were being serviced, Glenn checked everything out and replaced some parts for good measure. I was very pleased to know that they were still in perfect working order.

The QUAD II amplifiers were created by Peter Walker in 1953 and the build quality of these old Quads is simply wonderful, but they do need some space around them — they can get very warm. I was told they were manufactured back in the early 1960s which was nice to know.

When I got them back from Glenn, I powered them up and sat down to listen. I couldn't believe how much bloom and life the Tannoys gave with these little things in charge. After about 30 minutes of warm‑up, it was time to really try them out and give them some 'V'. This is the point where the LEAK TL12's really fell apart — they didn't like demanding music at higher volumes, so with that in mind I started with some Spiritualized, Deep Purple, and Wolfmother.

Wow ... straight away, the Tannoys sang, pushing out a full range of psychedelic dynamic goodness. Powerful and authoritative, I was a very happy bunny indeed. Playing ambient, Goa and dance also brought a smile to my face. The bass was great — not as focused as I would have liked, but still very nice.

The next step was to set out to buy some top‑notch vintage valves.

My first choices were Genelex or Brimar, but I finally fell on some original 1950s GEC KT88s and signal valves.

Once the GECs were in the amplifiers, things improved even more. I was so surprised. The bass was now more focused and had more authority whilst the mids were slightly leaned back. This wasn't a bad thing as it brought out everything with more ambience.

Another thing I found with the QUADs was that given a 1‑hour+ warm‑up, things really started to boogie.

Another worthwhile upgrade was using the Mark Grant interconnects together with some Van Den Hul The Wind MKII Hybrid speaker cable. I found Mark Grant cables really gave the QUADs a more natural sound — highly recommended.

To my knowledge, these QUAD II's were a one‑off custom job from Croft Acoustics. But I would recommend having the same modifications done to any old and tired QUAD II's. You will certainly fall in love with these little anvil‑like wonders again.

Once the GEC valves were installed, the QUAD II amplifiers reached a new level of refinement. The bass gained authority, the midrange opened up with more ambience, and the overall presentation became more atmospheric and engaging.

With over an hour of warm‑up, the QUADs really came alive. Their synergy with the Tannoy Monitor Golds was unmistakable — a pairing that delivered musicality, bloom and dynamic expression in a way that modern amplifiers often struggle to replicate.

Upgrading the interconnects to Mark Grant cables and using Van Den Hul The Wind MKII Hybrid speaker cable further enhanced the natural, open sound of the system. Highly recommended.

Frequently Asked Questions

Are the QUAD II monoblock amplifiers still good today?

Absolutely. Even decades after their original release, the QUAD II monoblocks remain some of the most musical and engaging valve amplifiers ever produced. Their timeless design, rich tonal balance and compatibility with high-sensitivity loudspeakers make them highly desirable among vintage hi-fi enthusiasts.

What modifications did Glenn Croft make to these QUAD II amplifiers?

These particular QUAD II monoblocks were modified by Glenn Croft to run KT88 output valves instead of the original KT66 valves. The amplifiers also received bespoke circuit upgrades and servicing work designed to improve reliability, dynamics and overall sonic performance.

Do QUAD II amplifiers work well with Tannoy Monitor Gold speakers?

Yes — the pairing between QUAD II monoblocks and Tannoy Monitor Gold loudspeakers is exceptional. The amplifiers deliver excellent midrange presence, smooth treble and a wonderfully spacious soundstage that perfectly complements vintage Tannoy dual-concentric drivers.

Do vintage valve amplifiers require warm-up time?

Definitely. The QUAD II amplifiers improved noticeably after around 30 minutes to one hour of operation. Once fully warmed through, the sound became fuller, more dynamic and considerably more immersive.

Did changing to vintage GEC KT88 valves improve the sound?

Very much so. Installing original vintage GEC KT88 valves brought tighter bass, greater authority and a more atmospheric presentation. The upgrade also improved overall musicality and helped the amplifiers sound more refined and effortless.

About the Author

Reviewed by Josie - audiophile equipment reviewer with over 30 years of experience reviewing analogue and digital playback systems, valve amplification, vintage loudspeakers and professional studio equipment.