The Topping pre90 is PIA to try because it has too much gain. It works with amps like the Parasound A21+ and Benchmark AHB2 which have adjustable GAIN. I ended up getting rid of it after it did not match with a lot of amps including the LSA Voyager 350 GAN.
The 2 preamps I recommend with all the GAN amps I have had and also all amps are the Holo Serene and Benchmark LA4. I also like the new Schitt Mjolnir (sp?). It is a headphone amp but also works perfectly in 2-channel. I have it setup as 2-channel but for for headphones. It has a Class A sound to it, unlike cleaner sounding Serene and LA4.
I did love what the Mjolnir does with the Class A Schitt Aegir amp into my phones. As a frame of reference, I feel the Mjolnir is way better than the Schitt Freya+ (I sold the Freya+).
The best GAN amp I have had and likely better than any GAN amp is the PeachTree GAN1. This piece is definitely leagues better than any GAN amp I have had, the PeachTree GAN400 and LSA Voyager 350 GAN, Now the stock version is not anything to shout about but get a $500 modded version from TweakAudio.com and you are in business.
No preamp, no DAC, no interconnects, no amp needed. It is all in the GAN1 but it only takes a SPDIF stream as input and has no volume control. Of course, this will work perfectly if you use the correct tools. The simplicity of this unit is what makes it something special, again I AM NOT TALKING STOCK.