Take a look at the Canary products, especially the CA-801 and up. Great sound, and really well built. I haven't heard any Supratek products, so I can't compare, but on their own merits Canary are a class act. Used units would be within your price range.
I don't know if you're open to other approaches than tubes at this point, but a transformer passive can be a very attractive alternative. I have a Canary 801 as well as an Audion Premier (a 6922 line stage with SS power supply), but with my Canary 339 monoblocks the Bent Audio NOH passive is noticeably better than either of them. However, on some other amps the situation is reversed, so this isn't an absolute recommendation, just something to think about.
I don't know if you're open to other approaches than tubes at this point, but a transformer passive can be a very attractive alternative. I have a Canary 801 as well as an Audion Premier (a 6922 line stage with SS power supply), but with my Canary 339 monoblocks the Bent Audio NOH passive is noticeably better than either of them. However, on some other amps the situation is reversed, so this isn't an absolute recommendation, just something to think about.