I've owned a couple of Markbass amps. Currently I'm using a TA503 and for the first time ever I'm happy with my rig! I've not experienced the older stuff but I've played the majority of the new stuff (except the REALLY new amps) and I've been very impressed with all of it.
The general idea behind Markbass is for the amps to not colour your tone and give you a lot of volume from a light, small package (ohh err missus!). It's incredibly reliable gear and does the job very well IMO.