Your current GK combo is way more powerfull, it already delivers 350 watts now with no extra cab linked to it. If you add an extra 8ohms cab, you can get 500 watts total.
The Markbass LM250 will never do better than 250 watts, you did not specified the Schroeder's impedance too: if it's a 4 ohms cab, then 250 watts. But if it's only 8 ohms the Markbass LM250 will only deliver...
150 watts.
Meh.
You'll loose power whatsoever if you choose that LM250 + Shoroeder cab combination.
Now, if you need something clean with a vintage option, the Markbass has the Vintage Low Emulator, VLE, filter that does a really good job mimicking a kind of P with Flats plugged into a B15 Motown sound.
As others have mentioned here, you did not tell us what REALLY "not that much modern/clean sound" is to you, like they've said I'd rather test and buy a preamp pedal, some of them even have tubes, like the Markbass Vintage Pre. I've tested it, it does the job, but it's pricey: around the 350 bucks mark.
Also, what's your budget ? if you plan on selling your GK combo to buy something else, then you'll be pretty limited. A good bass preamp starts at 200 bucks new and can go up to 500 ! you should have a look to the classifieds then.
Plus swaping a GK for a Markbass amp/combo will give you something clean and modern still, the only option with the Markbass is the VLE filter. Don't know if it's really worth loosing 100 to 200 watts power, only you can tell if it's worth. To me selling something else and buying a preamp in the classifieds would be a better option.
I currently use a Markbass head and have used the GK MB210 II, so I know what they can do.