I've used a bunch of amps for bass over the years...Let's see.
Mackie M1400i (Bought this guy in 96 I believe. My first pro amp and it just had a channel blow. Get this, it was being used on a JBL compression driver at the time. Figures .) I think the HF may have actually killed it. Can't really complain after getting 15+ years out of it. Need to pick up something to replace it but I'm not decided on what yet.
Behringer EP4000's (Still have 2)
Crown CE4000's (Sold)
Crest 8002's (Still have 4 of these.)
QSC PL9.0's (Sold)
AETechron 7560's (Sold)
Powersoft K10's (Sennheiser KPA2400's)
I've used a bunch of stuff off and on over the years that I didn't own as well. Tons of Crest, Crown, QSC, Samson, Mackie, Ashly, plate amps, etc...I don't have a lot of experience with consumer amps other than some Adcoms, Emotiva's and NAD's and I never tried them seriously on bass duties.
Some were good some not so good. The PL9.0's were really powerful but they were prone to blowing up randomly plus they were loud as hell with 4 fans in each. Bang for the buck it is still difficult to top the EP's. The 8002's are great, just a little better than the CE4000's were but they are big and heavy. The ridiculous award goes to the 7560's they were beasts. They weren't the most powerful but you could strap them together and literally weld with them and they wouldn't complain. They got really hot though and at basically 100lbs each they really just weren't practical.They were also amazingly well engineered and perfectly operating after 30 years of use. Not many products are built like that anymore. They were obviously an industrial tool not a mass produced commodity item. My favorite amp is the rebadged Powersoft K10's. At first I was skeptical about all of the claims but I'm firmly a believer now. Plugs into anything between 95-265v, PFC, 95% efficient, 1 rack space, 200v peak output per channel, password protected user control interface, less than 30lbs, relatively quiet squirrel cage fans and protected from just about every sort of load or ac supply issue. Unfortunately they are just bloody awful expensive. I never would have bought them if I didn't get such an insane deal at the right time. I can't seem to find anything comparable for cheaper either. I need a couple of the newer lightweight DSP amps but every one of the cost effective ones has something about it that bothers me. I should probably just buy a couple of the 4 channel clones but the blatant rip off aspect bugs me truthfully.
Generally I look for the following...
1.) PFC it really helps boost the efficiency and drag the current out of even a weak circuit.
2.) Universal Voltage supply. This is a major plus for me. Combined with PFC it means you can get the maximum out of whatever circuit is available and you are virtually guaranteed to have a compatible circuit.
3.) High efficiency topology, again relates to less heat, weight and more power
4.) extended response (No 10-15Hz filters please. I can add them myself if I need them.)
5.) Warranty & Cost
6.) Internal DSP (If needed, I don't yet consider this a must)
From there I start looking at things that I consider of less concern. Weight, size, cosmetics, fan noise, connectors, input sensitivity,etc.
Obviously looking at my priorities versus DS-21's for example, there are some differences. I don't think that I have a single RCA cable involved in my main HT. Everything is xlr or hdmi. Everyone's priorities are different. I do pro audio type things often so those are these types of amps that I gravitate towards. I am starting to become interested in some of the plate amps on the market. Speakerpower and Pascal in particular. I wish that there were a little more user control and functionality in some of the cheaper plate amps. Hopefully DSP will start to be included more.
It'd be nice if there was a 1000w plate that has user configurable DSP with boost and configurable / defeatable high pass filter for $500 ish...