Since you are considering one of the above power amps you might want to check out the new QSC PLX 2 series; pretty light and cleaner sounding than the Stewart.
Wow, Jorg, I'm surprised to hear this... I was really less than impressed with the PLX (albeit it wasn't a PLX2)...
I compared a CA9 to a PLX 2402, and a Stewart 2.1. Here's a summary of what I felt about them...
The CA9: This is THE definitive bass rig power amp IMHO. Seemingly endless headroom and boundless tone. However, every time I picked it up, I literally couldn't believe how heavy it felt. This ended up being a deal killer for me. Other than the weight, the perfect amp. These are known to be practically indestructable from a construction quality standpoint, and are one of the most road-worthy amps out there. Worry-free and incredibly powerful.
The Stewart 2.1: IMHO, very similar to the CA9 in as far as it has a very "alive" sound to it, very broad tone, great power, and incredible portability due to the weight (tho I wish it would fit a shallow rack). My one niggle: the A/C power cycle time. If A/C power goes out to the amp, you'll wait up to a minute for it to come back on, and they don't like to be plugged into line filters. It all has to do with the switching power supply design. That said, however, I've gigged with this thing MANY times and LOVED it. Works with my TFB and my Edens used as preamps... had a BBE BMax-T preamp and it sounded fantastic with that, as well.
If you want a lightweight power amp, IMHO, this is the one to get.
PLX-2402: GREAT feature set, but "lifeless" to my ears. Maybe this means it's actually more "accurate", but IMHO, it's way better suited for PA than a bass rig. I really wanted to like this amp due to the price I got, the features, the weight, and the fact that it also fits a shallow rack. However, A/B'd with the Stewart, the Stewart mopped the floor with it somehow. It went back.