DACs that do well without a preamp

a DAC that uses transistors, or tubes for the output stage is not passive, it may not have gain like a preamp, but it's still has an active device for the output.

While that may be true, many of them do not provide enough gain to fully drive an amplifier, which is the primary issue in many cases.  Active or not. 

This Bel Canto e.One DAC/Pre is definitely worth a look. Bel Canto makes excellent-sounding DACs — I had their DAC3 VB in my system and its sound was near flawless to my ears. But what was even more surprising was how well it performed as a preamp. When comparing it directly with my $2400 Bryston BP-6 stereo preamp I literally couldn’t tell the difference between the two. Not sure how many other DACs could pull that off, but I’d bet not many especially in your price range. BTW, I was using a McCormack DNA 0.5 RevA (getting it back Saturday with full SMcAudio upgrades) and it was an excellent combination.  Hope this helps, and best of luck.

https://www.audiogon.com/listings/lisb10dg-bel-canto-dac-preamp-da-converters

I am baffled as to why so many otherwise high-end DACs sound so poor when connected directly to a good power amp.

Frequently it’s just that the passive preamp sections usually included in the DACs don’t have sufficient output to fully drive the amp it’s hooked up to. As always, it comes down to proper system matching. Personally, I’ll always use a separate stereo pre as I find it to be a critical component to get the best sound out of my system. But that’s me.