Hi -- thought I'd add my 2 cents. I was using a Crate Power Block 150 (150 Watts bridged, Class D digital amp, about the size of a loaf of bread, and about 5 or 6 lbs) for about 5 or 6 years, but just picked up a Crown XLS1000 rack mount power amp. I found I needed more stage volume in different situations.
Played a fair pavillion this summer, crazy loud crowd, and I could barely hear myself over the din of ~3000 people.... Anyway, also have the problem in a lot of outside gigs, where your sound just evaporates. For one gig, I took my backup Crate PB150 and ran my practice speaker -- identical to my gigging speaker. Both are Eminence Tonker 12" in old floor monitors rated 150 Watts each. With the 2 amps & speakers running, I had good levels. I did not run twice as loud, but was no longer "topping out" the single 150 Watt amp -- you can hear some bad clipping when you do that....
I run the XLR of my POD-HD500 to the soundman's FOH system. It got me thinking about the rackmount power amp concept. The Crown XLS1000 runs 215 Watts per channel into 8 Ohms, 350 W into 4 Ohms, and 550 W into 2 Ohms. If I run my single speaker (8 Ohm) on 1 channel, I have 215 Watts, compared to my PB150's 150 Watts.The speaker can take it. When I get into the festival/outside setting, I'll also take my practice speaker and run either Y mode (same mono signal to both sides) or go stereo from the HD500.
The rack-mounted amps can take a lot of different speaker configurations. Running stereo or Y-mono, I'd have ~430 Watts available,but would turn it down and get a lot of headroom -- running nowhere near that actual wattage. I put the XLS1000 in a softside RoadRunner 2 space rack case. The XLS1000 is only 8.9 lbs, and in the softside rack, about the weight of a briefcase. I still take my Crate as a backup for contingencies... Just like having a 2nd guitar sitting on the rack..
The Crown is lightweight, and not hugely expensive, around $300, but I got it on sale at Guitar Center this Labor Day for $250. It is a Class D digital amp, and is very transparent, and clean. No digital artifacts, or fizziness. It likes line level, so I run it on the unbalanced 1/4" outputs of the HD500 at Line level. The XLR balanced outs don't give you enough gain to begin with, so you'd need to run it into a little mixer to boost the signal level. The 1/4" in's of the XLS are balanced TRS (Tip/Ring/Sleeve), but work fine with unbalanced TS coming from the HD500.
Anyway, that's my take on the Crown XLS1000 -- very workable for a guitar rig.