What does moving from a 150 watt amp to a 400 watt amp get you?

Thanks all for the incredibly helpful information (even the humorous responses!).  It is doubly dangerous to be armed with science AND personal experiences.  

Nutty / Aalenik - I'm using an ARC Ref5SE preamp currently.  It took me about a week, maybe more of long listening sessions to fall in love with it.  Initially I was a bit disappointed in the changes in moving from the K-5Xe - but by the end of two weeks couldn't move back.  

Overall I'm pretty pleased with the way things are sounding now.  Occasionally there is a bit of harshness in some of the highs - but it is minor.  The best systems I have heard make me want to keep nudging the volume up as I listen.  Mine seems to have a point at which louder is not better.  I don't think I'm listening at ear-splitting levels mind you - generally when listening "loud" it rarely exceeds 82 to 83 db.

I use thousands of watts to provide clear sound for concerts I produce/mix, but have become part of the "Lower powered single ended tube amp cult," LPSEAC or as commonly called "lipseek" (you've seen the t-shirts, right?). By joining the cult you agree to a couple of things: You use efficient speakers (99db in my case, plus hundreds of watts in powered subs), and you collect tubes. For many decades (I'm 106) I've owned tubeless and tubed hifi amps with from 60 to 250 watts per side, biased class A-ish for at least the first few watts or something, and have enjoyed music spilling out of these things for years…but after joining the lipseeks I'm stuck. Nothing to my taste sounds better, clearer, or more emotionally engaging than a properly utilized group of single ended glass bottles shoving electrons around. Headroom SCHMEADROOM I say (as long as you use efficient speakers or sit with drivers a foot from your head). I haven't tried an OTL amp…yet...

Thanks for the peace of mind. Yes, they are P3ESR's. I'm glad to know the amp is enough.. watts really are a funny measurement.

Indeed. Your amplifier is more than powerful enough for the P3ESR. In fact, you will need to exercise some caution. You are fine. You did not say how large the room is, but if the room is medium size or larger, the only thing you could do to get a 'bigger'sound is add two small subs like the SVS SB1000 and an Antimode 8033 (but don't add subs if the room is small).

@wolf_garcia  You haven't lived until you've heard a pair of original Futterman H3AA's with KLH Nines or Quad 57's! Sublime musicality rarely equaled!