I've gone 'Bloak Crazy, myself!(oops! - did I really say that?)
They are SOOOO sweet, easy to build, and cheap, even with Darlinton outputs(Sound better than BD139/140 output to me.)
Just try a handful, 'Bloaks, 2520's, Melcor, 990, Forsell, JLM, whatever. The fact is they ALL sound great to me, but are better at specific aplications. I thought that the 990 sounded clean 'til I tried a BigBloak! MAtch the flavour to the specific Application. I have 4 different types of Output transformer(2503, Audix, Gardner's and Sowter, and I have now built them in seperate small boxes with XLR in/out, so I can mix and match for different applications. I also have several different "versions" of a API-312-type Mic-pre, with different types of Input Iron(Marinair, Belclere, OEP, GArdner's, Sowter, Beyer and DOA's - not so easy to swap about! This makes a really flexible range of sound - AWESOME!
I think that YOU should decide which is best by trying them, not asking for our opinions, no-matter how valid they are! Build with whatever you can get hold of, and try some others when they come along. I was seriously biased against the humble Melcor 'til I actually tried it! I like them just as much as the 'Bloaks, yet they are two ends of the spectrum effectively.
"Clean" is a subjective matter if you ask me. Some tube gear sounds "clean" yet runs at an order of magnitude more distortion than Modern chips, which can sometimes sound quite muddy. Some tube gear sounds muddy too! Even MAssenburg gear has a "signature", and the specs there for distortion, Slew-rate etc, are pretty much second to none.
As to transformers being dirty, well of coarse they can be, but I agree with wot TKHalmi sez. Especially the lower step-up part. My pre's with Marinair wired 1:2 with BigBloak's and Audix output trannies rank amongst the cleanest I have ever used, especially at the top end. Bottom is still nice and warm.Best of both worlds! (Definately a bit more grit at 1:4 tho'). I feel that I have finally found the holy grail! I will never be able to afford the Massenburg pre's which would probably be my ultimate comercially available choice for many different uses. Not sure I would WANT to now, anyway! Unless I needed 70dB of gain . . . .
Enjoy!
ANdyP