I have done some testing in this space as the importer for Art Audio and one of the importers for KR. I have had A LOT of preamps here. Some from big companies. Some from little companies and in the end, it comes down to matching with your power amp and personal taste.
I ran the Art Audio Conductor for a stretch and it sounded better than any other preamp I have had here with Art Audio, VTL and Rogue Amps. When it arrived I had a PS Audio BHK and an ARC in on trade and a variety of others through time. It has switchable gain from an active circuit with no gain to 25dB of gain. So long as I was matched with a power amp where that made sense, the sound was incredible. Give me an amp that was in the middle and performed best with 12dB, results were not as good.
If the designer knows what he is doing and the components are well selected, a small company can do an incredible job. Often, you can get some really great designs or really different approaches that you won't see from a bigger company (KR P135?????) which can be awesome.
But, you have to vet them. Is there a service center? Are the parts really esoteric or is the equipment pretty mainstream? Are they truly making it by hand (point-to-point wired) or is it PCBs that are being assembled. Talk to certain manufacturers/importers and they will tell you point-to-point wired is the only acceptable method for production and PCBs are garbage. Talk to certain repair shop and they may disagree rather vehemently.
In the end, it comes down to the individual company. Unless it is silver wired, I look for amps and preamps with PCBs (not a lot of silver PCBs out there) where the output specs are what I need for my amp. That is where I start and then it comes down to the tube compliment to determine if I think I want it with my amp.
