Amps & VALVE Amps

Hopefully, one of those things you set-up and forget about. Cheaper ones especially have a propensity to melt. I don't have that much to say on amps, because their job is simple to understand.

Obviously, amps take in line-level signal and mains power and output high-power signals for the speakers. By their nature, class A/B amps are only around 15% efficient, outputting 85% of the input power as heat. So some care is needed...

CLIPPING

As described in the "Cabs" section, clipping is really bad for your speakers. If you can, use a firewall limiter to prevent the input signal clipping (described elsewhere), but, the last source of clipping is from the amps themselves. 
If the amp is running near full power, peaks in the dynamic range can cause the capacitor network to run out of power, and rather than amplifying the full peak, it cuts the top off; clipping. Strange as it may sound, it's far  better to have an amp more powerful than your speakers, run at a lower level, so when a 'peak' does occur, it's amplified correctly (~25% in excess). Granted. this will, momentarily, be over-powering the speaker, wheras a clipped signal will be under-powered. HOWEVER, the clip is DC and the peak is AC. The overall result, in terms of heat buildup on the voicecoil, is that AC is MUCH better. 

DAMPING FACTOR

The damping factor describes the amps ability to control bass notes (ie: tight and accurate). The higher the factor, the better, and it really only effects bass. Calculated by the output impedance of amp divided into impedance of speaker network connected to it. 
By speaker network, I mean the entire impedance presented to the output of the amp, inc. the cabling. Bad speaker cable can decrease damping factor, as well as robbing the expensive output power into heat, as can bad connectors: XLR and Speakons are low resistance, anyone who's found a jack welded into it's socket after a night will appreciate the high 'resistance' of jack connectors.