I guess I should qualify my statement about purpose built bass amps. For the most part, my experience is that they tend to be less power for the money than PA amps. I am referring to the power amp section of all in one heads. It might be that there are some power section only purpose built bass amps that have a fair amount of power.
I have an Eden WT1205, which is apparently the most powerful amp head they've made. It's still only 325 watts per channel into 8 ohms and they don't say at what amount of THD or at what frequency. My QSC is 700wpc at .05%THD at full bandwidth. It doesn't hold a candle to the Crown, Crest or QSC amps I've owned when it comes to headroom and oomph. For sheer audio performance, it's more cost effective to get an Eden Navigator preamp and then a PA power amp than it is to get the WT1205. On the other hand, the convenience of having it all in one package does factor into it if that's what you are looking for. I think that the fact that Alembic has brought two market two preamps but no heads speaks to the situation that there are plenty of power amps out there that do the job better than something that's all in one.
Another aspect is that if the power amp goes down, you can still use the preamp with a different amp. This affects routing options as well as recovering from failure. The WT1205 cannot access each power amp separately, so I can't use it to run stereo effects or a stereo bass all the way through. I was told I could. Major fail. My WT1205 blew up at its very first gig and I was without it for 17 months while trying to get it repaired (long story, they replaced it and I just took it out on a gig for the first time last night). So, if you can get a second hand Eden for a great price, it might be the thing to do, but don't assume that because it's discounted from the Eden retail price that it's the most economic option. Also, from my experience, Eden has a pretty serious reliability problem. Their customer service has restored a certain amount of my faith, but pretty much every piece of gear I've gotten from them has broken down. My Navigator is sitting broken as we speak. It can sound good while it works, but I wouldn't go to a gig without a backup. So far the warranty replacements I've gotten for the WXT 500 and WT1205 seem solid. The Navigator was a warranty replacement but it has failed twice now. I don't want to speak ill of them completely because I've heard that they were very reliable up until about 2005 or so.
Also consider that bass amps are a smaller market so fewer are made, which results in a worse economy of scale and also the fact that PA power amp companies make a ton of them gives them a lot more experience and opportunity to experiment with design and manufacture. If Eden hadn't rushed the WT1205 to market and had been in a position to sell thousands of them, it would probably have been a better design without the manufacturing issues. It only takes one bad batch of parts to mess up a manufacturing run and then you've got a much higher percentage of your product on the street failing and your reputation goes down, sales go down and then it gets worse from there.
