Class-A Amplification and 15 Amp Outlet

Look at the idle and power rating for the amps. A 15A outlet is rated for about 1800 VA I believe, so long as the amp plus your accessories stay above this you stay well below that you’ll be fine.

You might want to swap to a better outlet though, especially if this is an old house. See more here:

You may also want to consider a conditioner with voltage regulation to ensure the voltage doesn't sag:

Of course, you can  also have a dedicated circuit with 12 gauge wiring and convert the line to 20 Amps. The beefier wiring, above all, will improve power delivery under stress.  If your circuit is going to be over 1,000 VA most of the time I'd suggest it.

Watts devided by volts = amp draw.  The Pass XA160 you mention draws 600 watts, which equates to 5 amps (600 watt devided by 120 volt power supply).  My entire system (Hegel H590 amp) draws less than 2 amps when playing music.  You should be fine on a 15 amp breaker but you need to calculate the total amps required before choosing equipment? 

I was told by Pass that my 15A  lines were fine for the XA160.8 mono blocks, and after 2 years they definitely are just fine...

Just plug them in and play them.  We use a Walmart power strip for around $10 and it works fine.  You are better off spending money on upgrades capacitors, resistors, chokes, etc.