Hypex UcD400ST, digital amp
Copyright 2013-20 © Troels Gravesen

Not bi-amping larger 3-way systems seems almost silly! The back-EMF generated by large bass driver voice coils can make many amplifiers uncomfortable - in particular valve amps - and impair midrange and treble performance. More from Wikipedia on the phenomenon here.
A power amp doing 100 wpc can these days be had for almost nothing, e.g. Behringer A500, which retails for less than 200 € on these shores. This is so cheap we could use the chassis and heat sinks alone and mount our own modules in case we have better ideas. Reviews of the A500 amplifier doesn't exactly suggest stellar performance in midrange and treble, but who cares for an amp supposed to do nothing above 200-400 Hz. What matters is its ability to deliver some amperes.
I decided to go a little further aiming at 200 wpc and the Hypex modules seems ideal for this. Ready made circuit boards, and all you need is a chassis and a bunch of wires. I bought a 625 VA toroidal transformer locally plus a small 2 x 15 volt transformer also needed for the UcD400 module, which I wasn't aware of at the time I ordered the kit. Initially I wanted 2 x 250 VA transformers but couldn't find these with 2 x 40 V secondary windings, hence a single 625 VA transformer.
It has to said that drilling more than 150 holes in the alu sheets took a couple of evenings, but I wanted the chassis to serve as a base for my GlowMaster KT88 valve amp, thus 32 cm wide and 40 cm deep and getting the chassis painted by a pro wasn't exactly cheap either, but what can you expect when you ask professionals to paint a single chassis?

What I was most anxious about was how the gain of the Hypex amp would match my GlowMaster KT88. Fortunately the Hypex turned out to deliver + 1 dB compared to the GlowMaster when run in parallel by my Yoshino EAR-912 or Yoshino EAR-868PL. I was expecting a little more and have added an attenuator (2Px6) on the rear panel of the Hypex allowing 0, -1, -2, -3, -4, -5 and -6 dB attenuation.
The Hypex is claimed having a voltage gain of 20 = 26 dB. I measure 25.6 dB. Anyway, as-is the Hypex is the perfect partner for my EAR or GlowMaster power amps and delivers a firm, solid bass from my reference speakers. Even the DTQWTs don't object to this combo despite seriously overkill in terms of power requirement.
My ordering list was this: 2 x UcD Supply HG, 2 x UcD400ST, 1 x UcD Softstart, 2 x Neutrik XLR female 3 pol, 1 x Switch Push Button. The rest I bought locally. I find no reason to buy the possible better sounding versions of the UcD400, the UcD400HG (high grade) for bass application only. Today (2020) I would have bought the FUSION modules, but I have no intention of replacement. The UcDs work wonder here.

My resistor choice to make max -5 dB gain did not exactly turn out the way I wanted, but the attenuator works in parallel to the input shunt resistor and I didn't want to fiddle with smd components.
Above the actual measurements displaying the reduction in gain for the 5 stages.
However, this suits me quite nicely and I'll leave it there.