Anyone experienced with Coincident 300B?

Yes, I have heard them several times and on different speakers. They are quite nice amps, particularly for the price. You should be aware, however, that these amplifiers are made by ASL and are fancied up with better cosmetics and (reportedly) better parts. This does not mean anything in terms of sound quality, of course, but it is interesting to know.

I use an SIP-300 in my headphone system powering AKG K-1000s.

My amp is dead quiet. Digital bias is finicky. I'm not frying my 300Bs yet, so I think I'm correctly interpreting the bouncing readout. Very nice sound. Some folks on these forums rank PSET far below push-pull and SET in 300B sound quality preference. The only other 300B amp I've owned was a much pricier/trendier 300BXLS SET amp that was way too forward in its midrange presentation (so I sold it).