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

I don't see why an average sensitivity speaker (say 88 to 91 dB/Watt) with an impedance that reaches a low of 3-4 ohms would need a 500-watt amplifier. Every doubling of the output level requires a tenfold increase in the power delivered to the speaker. So, let's assume we have a speaker with a 90dB/watt sensitivity. 10 watts will result in a 100 dB SPL (sound pressure level, the actual measure used to describe the loudness we hear). A 100 watts will result in 110 dB SPL. Considering that most amplifiers can supply, for a very brief period, double their RMS, or continuous, power output, a 200-watt brief signal will result in nearly a 115 dB SPL sound level. Other than using your rig to make people deaf, or induce tinnitus in their ears, or develop some really bad relationships with your neighbors, there are very few musical events that go this loud. Maybe the very highest peaks of a symphony will reach these levels. So I will always vote for quality over quantity, provided that the amplifier I choose will meet my basic requirements for reproducing what are to my ears, loud enough peak SPL's.

Invite a 170 pound 60 year old to hit you like a football tackle. Then invite a 260 pound college football player to hit you like a football tackle. Take the data generated and apply tangentially to amps.  

Glad I could clarity that for you.  ;)  

It's all about sensitivity and headroom. If your speakers make 90db with one watt. Then they will use one watt no mater if you have a 50 watt or 500 watts amp connected to them . The only difference is you paid a lot for those extra 499 watts that your not using. You need headroom for the loud spikes especially in classical music but 15db should be more than enough. So to get that headroom you need your amp will need  to deliver 16watts. Quality watts are expensive.  Doubling an amps power often means a doubling any of its problems plus a quadrupling of the price. If you don't need them then it's better to go with a lower powered amp with higher quality watts.

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

250 watts yuck yuck!

sorry, just couldn’t resist!!

POWER !   I had not heard my DQ 10s, not really - until I went from a 60w/ch tube amp to a 250w/ch ss amp.  It then rocked out! drums were heard AND felt, bass lines became distinct, dynamics went through the roof.  It became capable of inducing FEAR also when in a film we were watching had some low, threatening sounds played.  

Let none tell you it doesn't matter, my friend.