Need recommendations for high gain/small footprint class d amplifier

If you decide to go with Hypex NCORE like Buckeye, spend a little ($300) more to get NCOREx line such as NCx500 instead of NC502MP. The amplification efficiency (A.E.) is better for handling lower impedance loads

https://www.buckeyeamp.com/shop/amplifiers/hypex/ncx500/2_channel

Speaker Imp. NC502MP  A.E.    NCx500    A.E.

2 ohm            450 watts  32%    700 watts  46%

4 ohm            500 watts  71%    700 watts  92%

8 ohm            350 watts    -        380 watts    -

Price                 $695                     $995

As shown, NCx500 will handle 4 ohm load speaker much better.  The amp efficiency from both models for 2 ohm load are still not great but not bad for handling not so demanding low imp. speakers for the price.

@lcherepkai in my custom speakers, which I was told should be about a 3 ohm load.

It seems like you never measure your custom speakers. Try using REW to measure the impedance-phase angle versus frequency yourself, and then compute the EPDR using a more refined empirical equation (from Stereophile) that I derived in my post in the following thread. EPDR will reveal the most critical load impedance you are likely to encounter. For nominally 3-ohm-rated speakers, depending on the phase angle, you will likely face impedances below 2 ohms in the mid- or upper-bass frequency range. This is why I strongly urge you to carefully consider the amplification efficiency of the power amplifier when dealing with speakers rated for a 2-ohm load.

https://forum.audiogon.com/discussions/impedance-the-most-ignored-and-useful-measurement-tool

My Buchardt S400 MKII is rated at 4 ohms, but the EPDR drops as low as 1.5 ohms at 110Hz—very demanding. Even my Parasound 125-150 watt high-current power amplifier (35-45 amps peak) could not drive it efficiently. I need to place the speaker closer to the front wall to achieve better bass response, but this comes at the cost of compromising soundstage depth. Check out the impedance-phase angle and EPDR vs. frequency chart posted for S400 II by Erin, and you’ll see what I’m talking about.

I think your best choice so far (if you can or are willing to afford it) would be the Benchmark AHB2, which has dimensions of 11.04" W x 3.47" H x 8.33" D and a gain ratio close to what you’ve been considering.

29.3dB (8 ohm) -> 29.1dB (3 ohm) -> 28.07 (2 ohm) (note: converted from voltage gain Benchmark has published)

More importantly, in my opinion, is the amplifier’s efficiency as I advocate earlier (see below). This is the best specification I have seen from a reasonably priced ($3,500) yet highly respected amplifier. The only amplifier I’ve come across with up to 100% efficiency at a 2-ohm load is Accuphase class A series such as A80, which could cost you more than $10k. Almost all Hypex- or Purifi-based class D amplifiers fall short in this regard at 2 ohm load impedances.

• 100 Watts per channel into 8 Ohms, both channels driven
• 130 Watts per channel into 6 Ohms, both channels driven (97.5%)
• 190 Watts per channel into 4 Ohms, both channels driven (95%)
• 240 Watts per channel into 3 Ohms, both channels driven (90%)
• 259 Watts per channel into 2 Ohms, both channels driven (64.8%)