RWA Isabellina HPA OR TTVJ Apex /Volcano

These are quite expensive - shouldn't you consider trying to demo them first? 

Add Cavalli Audio Liquid Fire to the top of that list

FWIW, when you buy the RWA Isabellina, a lot of the purchase price is going for the DAC.

If you already have a DAC you are happy with, then I'd probably recommend the Apex.

If you don't. Then RWA.

Tmoney's on the money: If you already have a great DAC, get the Peak. You can have some fun rolling tubes which have a very large affect on its sound. You'd be surprised with what vintage tubes you can find between $30-$70 if you do some research. Or you can just blow a few hundred on the rare ones that are known to be good.

I own the Peak/Volcano and have had a chance to hear the Isabellina HPA a couple of times. If I had the room or just extra money to burn, I'd get both. So here's some no B.S. impressions.:

The Peak/Volcano when paired with a great DAC (and you do need a good one here) is very dynamic, transparent, and resolving. It's gets all the micro details and micro dynamics right. If you choose, you can get a lot of bass kick and slam with the right tube. The treble is very extended - some here on Head-Fi have even said bright - although personally I don't think it's bright sounding. Currently I'm using the Peak/Volcano with the HD800 and HP2 with two different tubes depending upon the headphone. It's a really nice being able to easily swap tubes depending upon the headphone you are using. The HD800 out of the Peak/Volcano can compete (some things better, some things worse) with the STAX SRM323->SR507 combo, and that says a lot.

Where the Isabellina HPA shines is in the tonality department. The tonality and overall presentation of every headphone I've heard out of it (HD800s, LCD-2s, HP2s, etc.) has been nothing short of outstanding. Also, the HPA has probably got the blackest "blackground" I've ever heard from any amp - something that should be expected because of its battery power supply. This really gives music that groove and oomph for lack of a better words. The Isabellina also has the advantage (or disadvantage depending upon how you see it) of a built-in DAC section that's been voiced to mesh with the amp section. Vinnie Rossi (not that I really give a crap about who he is or what that name means) has voiced things very superbly. 

I've always considered myself a "resolution" (in favor of tonality) kind of guy, but the Isabellina, even with its NOS DAC has made me reconsider. If I had to do it all over again for a headphone only setup, I'd could easily pick Isabellina HPA over PWD->Peak/Volcano. Though admittedly, I don't think the PWD->Peak/Volcano combo is optimal. It's still pretty good, but not optimal (I think the PWD->STAX is a better match). 

Based on what I've heard, it's very hard to get both superb resolution and tonality at this price point. You've have to go up a little bit, maybe a Eddie Current 2A3 (see jp11801's review) or BA.