Low heat alternative to FIRSTWATT J2 ?

@almarg 
He did point out that it was a DIY effort and it's not at all uncommon to bias those things to the bare knuckle limit of what a pair of heatsinks will dissipate. That's exactly what I've done with my F5 to get 40 watts out of it, and I've seen builds where people have used absolutely gigantic sinks and biased the thing up over 2 amp. Generally FW amps sound best with as much bias as you can get away with. Nobody in their right mind would sell a build like I'm running. 

@pani 
The J2 and F6 are very different creatures. The voltage gain stage in the F6 is a transformer that drives MOSFETs in push-pull. The J2 is a pure JFET single ended design with somewhat limited current delivery capability on account of it's single output device. It likes higher impedance loads. Nelson talks about both these amps in his lectures at Burning Amp and the First Watt site has a wealth of reviews. Diyaudio.com is also rife with opinions on the various designs. 

@kosst_amojan, have you heard the J2 or F6 ? I am using a Tannoy Turnberry which is 93db efficient. Not really very efficient but still happy with 20 watts SET. The question is, will the J2 sound too relaxed compared to F6 ?

Kosst, good points in your response to my post.  Thanks!

Regards,
-- Al
 

@pani 
It's not so much the efficiency that will dictate the character of how they work together. It's more the impedance and phase angle. I'm not familiar with those specs for those speakers. Stereophile did a good review of the J2 and compares it's performance with a variety of different speakers. I'd imagine the J2 would drive almost anything that does well with SETs. The F6 is known to have a tube-like sound as well. These aren't the easiest amps to find and audition so the best you can really do is read the best reviews you can and get familiar with the reviewers broader body of work to get a flavor for what they're really hearing. I've never heard either, but I'm pretty convinced the J2 won't be an amp I ever build because of how it handles very reactive loads like my Focals.