Yes - I didn't make that very clear did I? Ha... I have two MAG 4x10's that I've used regularily for 3 or 4 years. I'm honestly at the point where I'm falling out of love with them...but that might have something to do with the SVT fridge that's sitting right in front of them right now

First thing I'll say is the MAG cabs needed acoustic damping on the internals. I finally got around to adding some poly batting (just bought a bag of the stuff they have at WalMart) on the inside and it was a HUGE improvement. The low midrange got a lot
more focused - not quite ABM quality but very very close.
As far as a comparison goes: All the Ampeg stuff I've played has been an array of 810's, some smaller combos like the B2 with the 4x8 cab that's at my rehearsal spot right now, and various other small cabs that I never really paid attention to what was in them. Everything was generally paired with an Ampeg amp though so I have no idea how much of this is the cab, and how much of this is the sound of the amp...but the common thread between them all is huuuge low end and a really sweet, chiming quality to the top end. The MAG cabs are quite a bit drier sounding - there is/can be a sweetness to the sound still, but it's in a different frequency range. In a way, the MAG cabs might be a bit less coloured sounding - but that might mean you have to work harder with eq to get the sound you want. Then again with my head, all I generally have to do is bump the low mids up a touch, and maybe lower the mids a bit, and I get my sound.
I've been going through a "hate my cabs - need some Ampeg" thing for the past two weeks btw, so I'm trying very hard to ignore that and give you an unbiased answer here.
This is just an educated guess on my part, since I don't have any experience with the SVT-410HE in particular - but judging from the experience I do have with Ampeg cabs and my Ashdown MAG's, you'd probably be mildly dissapointed with the tone of a single MAG 4x10 if things need to get loud. If stage volume is generally low then you'd probably do okay with one MAG - but personally I'd push for an ABM if that's an option. OR if this is a "the gear will just be at the venue when we show up" kind of situation, and you need the volume then ask for two MAG cabs

I don't really know what the situation is for touring for you - if bringing your cab causes problems across the board - ie, forcing the band to upgrade to a larger vehicle, or getting the band into situations where you might get turned away at the border (sorry I'm not familiar with how the state borders work in Oz) so I can't comment on whether any of this is better logistically than just bringing your Ampeg.