For $850 there are much better choices out there if you go used.
When you say "very loud" what do you mean? What kind of band do you play in and what sort of venues do you play (ie what kind of PA support is there, etc...)?
For almost all applications, 300-500 watts of power into a 4x10 or 2x15 should be plenty. A good/high quality setup like that would definitely beat the pants off your TNT.
Marshall has never been known for their bass amps really (although some of their vintage stuff works well for more old school rock sounds), and the new Acoustic stuff seems kind of cheap.
Better brands to look at would be SWR, Trace Elliot, Ampeg (if you're into that), old Acoustic, older GK or certain new stuff (not Backline, Goldline, etc...), etc...
You'll get a lot farther if you buy used/older PRO gear, rather than new/er "budget" gear. Take a look on craigslist in your area and ebay as well for a few weeks and get a feel for what's out there, then do a bunch of research online about anything that piques your interest so you can make an informed choice.
For a "can't hardly go wrong" approach you could find an old GK 800RB and an SWR Goliath III or IV 4x10 (8ohm, so you can expand later if you need to). Could probably score both of those for $600-800 or so. Both are "standards" and workhorses that have served a lot of people very well for a lot of years. For a slightly different approach with more wattage and a tube pre, you could switch the GK head for an older SWR Bass 350 (or an SM series, like an SM-400 or SM-500).
While my Trace head is down for a bit, I've been falling back in love with my bass 350, and playing it through some of my different cabs.
