That is because your markbass 410 is reaching its mechanical limits, and gets flubby like that.
I have not played the ashdown, as they do not have any kind of presence near me, until I get to someplace like chicago. Could be worth checking out.
I don't think so; with slightly lower volumes and with a HEAVY bass boost (did it on purpose to check which of the two was the bottleneck) the cab doesn't fart, and I can see that the speakers are not excursing much. When they do fart, they move a lot more.
Similarly, if I zero the bass and VPF, and push the master, things get murky.
Mind ya it's no deal killer but I'd like things to work as good as they should. Could be that the big caps need replacing on the amp, it was made in 2006 IIRC.