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
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Were the KT88's broken in? 

Can you try the 400 to see if what you hear is life changing or maybe a changeling?

Tube amp sound is mostly like Ralph said transformers and driver circuits.


yes, mostly true i would say, but it is interesting that in an amp like a primaluna dialog hp that allows you to hear different power tubes with the same input stage and transformers -- the different character of various power tubes (el34/kt77, 6l6, 6550/kt88) is immediately noticeable, as is even different makes of tubes (chinese vs russian kt88’s for instance)

another point to add here is that a bit of an oddball amp (but an utterly brilliant one) is a david berning (linear tube audio) zotl amp which is devoid of output transformers... i haven’t messed with one of these OTL since 20 years ago with the ginormous fourier triomphe otl’s ... but the zotl40 ref i recently got in is a revelation, so clean clear and extended top and bottom with lush mids and wonderful imaging (and so small and light in form factor)... very very impressed with what this little company (lta) has done...

70W vs 42W is only 2.2dB. You can almost get that by leaning forward in your chair. 

I got a DPHP because at the time my speakers were lower sensitivity and I wanted a little extra headroom. I was in denial thinking I’d want broadcast volume thump available from a tube amp when the reality is I rarely push it above 90dB any more which could be done on 10W. Plus I still have my old higher power SS integrated if I’m aiming to do damage rather than just listening. 

Since I was new to tubes, I didn’t fully understand how expensive it is to roll power tubes when you have 8 of them or what I would enjoy most from its character. I doubt I’ll ever feel a burning desire for KT150s just for the novelty at a cost of $600+. It cuts down on the experimentation side of things which was one of the reasons I wanted a tube amp in the first place. Hoping Santa brings me some GL KT77s to compare to the 7581As I’m using now. 

In my opinion, it’s about matching speaker sensitivity to the amp.  That said, I have found that “higher power” amps, especially solid state, you have to search for the sweet spot on the volume knob. That point where every instrument is playing at its best. I currently drive high sensitivity speakers with 6 watts of set power and It is perfection, at least to my ears. Not just at the sweet spot but exactly what you are experiencing, at lower volume levels. If you are thinking about a less efficient speaker, something like a maggie you will need lots more current and watts.  Had some 3.5r’s and found them as sensitive as a rock, they needed lots of juice.  Sounded great but needed the proper amp. If you are going to stay with what you have, for a while, don’t rock the boat. That said if your next step is falling for a different speaker, no matter the efficiency, cross that bridge when you get to it. 
To the OP.
Much good advice given already.  Seems like you're in a sweet spot, where you have an integrated that is designed to handle double the tube complement. 

But as you say " volume rarely goes past 9:30/10 " I sort of cringe. Aside from being meaningless, you really don't know if the amp craps out at
"11:00 or 12:00" or wherever.  You may be playing at 4 watts, in that case a 3db increase (double the perceived volume) would cause the amp to output 40 watts, ten fold increase.  Seems to me I would want to know where the amp STARTS to sound overdriven. 

This is anathema to many audiophiles, but I like a lot of headroom.  Give me 500 watt mono-blocks to play at 75 watts and bounce up to who knows where.

After all that if your rig thrills you everyday, you've hit the grand slam.
Don't change a thing!