I'll confirm that the A6 Avant can swallow almost anything. A bass will still need to sit in the boot at an angle, I've found - I don't think any of the modern estates have boots sufficiently deep to just slide it in, without needing to drop a seat. Angle your case across the boot, and space to spare, however. I'm easily fitting bass rig and the odd bandmate in without struggling overly, and the extra space on the A4 is a boost. That said, I've done much the same in the wife's A4 estate - and been a lot less worried about scraping the furnishings!
Of course, as Audi, Skoda, VW, etc, are all very similar you'll find the same kind of space on things like the A4, Passat, Octavia, etc - size will vary slightly by model, mind.
Also, brilliant drive and superb fuel economy on the diesels. They've just updated the rear, so you can find <2010 models going cheap, and most come with all kinds of gadgets.
Of course, if you're just wanting something to chuck the gear in, you could find something a [i]lot[/i] cheaper.