Output tube types - need some opinions

I'm starting to think about my next DIY project and would like to build a push-pull tube amp (monoblocks) with a single pair of output tubes. I do not plan on using any global feedback, but will probably include the option to run ultra-linear if the tubes support it. 

I don't have much experience with different tube types, so I would like to get some opinions about differences in SQ strengths and weaknesses of different types. I am currently using a pair of 300B parallel SET amps I finished last year (see my virtual system for some details) and like what they do. For this next amp, I'm hoping for more headroom (at least 30W), a bit better macro-dynamics, and maybe a touch less warmth. 

I'm not looking for kit suggestions. I plan to build these amps from scratch. But I don't want to work with voltages any higher than 600V which rules out some options. 

I listen mostly to jazz, folk, electronica, classic rock, and enjoy vocals. Excellent vocal performance is probably most important to me, but nice soundstage and decent imaging is also a priority. I use powered subs to cover the bottom couple octaves, so deep bass performance isn't too important, but good midbass performance is. My high frequency hearing isn't what it used to be, so extension out to 20khz and beyond will be lost on me. 

I've been thinking about using KT150s but I don't really know whether these are a good choice. I would appreciate some help deciding. Thanks.