Low heat alternative to FIRSTWATT J2 ?

Since you are so happy with it, I would try to make the J2 work somehow. Its not an easy amp to beat. But if you must, see is you can demo an Ayre V5. It does sound different from your Pass, but has a lot of the qualities you are looking for. There's a good chance that you'll like it better.

"I like the crazy good micro dynamics and rhythmic cohesion of the J2."

The Ayre does that better than the J2. It also has more resolution, more power, bigger dynamic contrast and the imaging is incredible. One thing that you may, or may not like about the V5 is that its not quite as forgiving as your current amp. Its probably due to the fact that Pass uses mosfet transistors, and most other amps use bipolar. You may think that's good or bad, depending on your personal preference.

Some other good choices you may want to look at are solid state BAT, Theta and maybe Sim. They're all fully balanced, 0 feedback designs, like your J2.

I would try to adapt to the heat. Is there any way you can sit at a distance that the amp is not annoying you with its heat, or are you concerned about children touching it.

Audio Consulting Mipa. 2x30W. 2 battery. No heat. I have one for sale. It needs a preamplifier with very low output impedance or passive with transformers.

The First Watts are sick good (I have the M2)…Man, I would keep it if at all possible.

Buy one of those small refrigerators and put the amp inside that. Cut a hole in it for the cables and seal it off with a rubber grommet. Problem solved. Just make sure you run a dedicated line directly to the fridge so its isolated from your system, and don't forget to put cones under it.