As above. I realized it is extremely hard a decent Dac that does 24/96 under $300.
I also prefer something that uses its own power sources and not usb powered DACs (such as the HRT series, Audioengine D1, etc).
The Emotiva XDA-1 is brilliant value at 249. It even supports up to 24/192 from every input apart from usb which is 16/44. Detailed with an open soundstage and splendid for less than 500 let alone 249. Just sold mine though as upgrading the DAC for my second/headphone system. However, I'll be spending a lot more in the form of a NAD M51 which runs about 2000.
sadly, i've checked with Emotiva and it is unavailable through retail in my country, and shipping direct from Emotiva adds another usd 60-70 to the cost :( Additionally, i read some issues regarding the gain as being too high. Is this still an issue?
For me anyways, It's looking like a £250-300 dac, which sadly is quite a bit more than $250
Probably the best one you will find for that money is the Jolida FX DAC. It uses two 12ax7 tubes with a capacitor output which are a common value and thus easy to exchange if you want a different sound. Sweet sounding little dac IMHO, I believe that it does do 24bit but I'd have to check though honestly anything that does 24bit in this price range is going to be more of a gimmic than anything. Just go for a really nice 16bit and you'll be happy, there is barely a difference between the two. For $90,000 Audio Note makes their DAC 5 Signature and that is only 18bit...once you hear it though, everything else just sounds broken. If you want an older DAC that sounds great go for a Theta cobalt (cheaper) or Theta DS Pro Basic. The Theta Basic will blow the doors off most modern DACs anyway, a friend of mine had one and I bought one to replace my Audio GD Ref 7 a while back. Very smooth rich sound with gobs of detail, the best part is the 3D space it creates. I would shy away from USB DACs at this price range. It will be hard to find one that is properly implemented and even those that are I would argue still sound worse than even an optical connection. If you must just get a converter from Audio gd and go USB out from your comp to coaxial into your DAC.
EDIT - the cambridge audio dac magic is also a great dac for the money, though a tad more expensive than your price range. All of the dacs i mentioned will fit your tubes very well. Seems like you have a little dot amp and these will add a bit more lushness to the experience.