Tube Amp lower power vs higher power and cruising volume


After having Solid State my entire life thus far, I bought a PrimaLuna EVO 300 integrated and absolutely love it. I am currently driving 20 year old B&W CDM9NT's and it does a wonderful job, I never heard my B&W's have that much bass before, even at lower listening volumes. The EVO 300 is rated at 42W. Since I recently purchased it I have the option for a brief time without losing money to possibly move up to the EVO 400 integrated which adds 4 more output tubes and gives you 70W. I believe the 300 and 400 both use the same transformer because they both weigh the same 68 lbs. So my only reason for possibly doing this would be for future speaker upgradability having a little more power. I know my B&W's are not the most efficient and the 300 seems to be driving them very well, volume rarely goes past 9:30/10. So my question, if I get a higher power model, since I listen most of the time at comfortable listening levels, with a higher power tube amp will you have to turn it up higher to hit that cruising speed where it starts to open up? The 300 seems to hit that early and I listen at comfortable levels and good extended bass without having to crank it which is nice when I am listening at night and my wife and daughter are sleeping. Overall I am very happy with the 300 but while I have the option I am trying to decide if the extra for a 400 is worth it. Thanks
128x128jmphotography
You also have the option of moving to KT-150 power tubes if additional power (and improved sound) is the goal.  Having said that if it were me I would always wonder and would probably end up moving up at some point.  I would move up if money is not an issue.  I own an HP Integrated with KT-150's and love it.
@testrun thanks. I tried KT88's and they were just too bright in my system, maybe its the B&W tweeter, so I am hesitant about any KT family as I seem to prefer that warm voluptuous EL34 sound.
I agree with those who recommend staying with what you have. You’re very happy with your sound quality performance, congratulations . I’d appreciate this good situation and continue to enjoy it. It’s ’possible’ that the additional 4 output tubes for increased power could potentially alter the fine amplifier and speaker synergy currently achieved. So yes, leave (Very) well enough alone. BTW I agree with the perspective that the el34 tube is likely a very good match for the B&W tweeter.
Charles
I would not move up for the sake of power unless I felt that my speakers were in need for more. To me the 4 additional tubes just complicates things. Very possibly providing more power without improving sound. Other's who have experience with both amps may chime in. I would only move up the line if it was proven to be that the more powerful amp sounded better,, and the money would not make greater improvement elsewhere in the system.

Running KT150s would improve power without additional tubes. Wether the sound is improved would be subjective. Would depend on the nature of improvements sought.

If I were seeking the sound provided by tube amplification (I am) I would not be in the market for speakers that are difficult to drive. Just my take based on preference and budget. 
I've owned a PL HP(Evo400) for 5 years. 

Since your not listening at concert levels, you're good. It's not a 4's good, 8 must be better thing. You probably would be fine even if you did like to put the volume at 1:00+. Buy different speakers(more efficient) if you feel the need to satisfy your nervosa. Roll the two innermost tubes for a subtle change, power cord, power conditioning, AC regen, fuse. If itch persists, get the EVO 400, or even better get separates.