MX7000 is bigger, probably newer and has active speakers compared to MX4000, so yes, it will be worth more - the problem with CRT tv's nowadays is that 2 * rather little is still rather little. In some cases you might have trouble giving them away...
I can understand why you would prefer a flat TV, but keep in mind that you will have to spend quite a lot of money before you get one that can even approach the picture quality (not to mention sound quality!) of both of these TVs. Most (non-B&O) flat TVs are barely enough for listening to news and completely hopeless for music unless you connect them to an audio system. I would at least keep the BV1, it is still rather "current" and an excellent set. The tube of the MX may be visibly worn already if it has been used a lot as you say.
And you could of course attach an MP3 player to the AUX input of your BS if you like it otherwise.
Scan eBay for finished deals to get an idea what these might be worth around your area. The TV prices can vary wildly since they are usually impossible to ship. The BS2500+BL2500 combo will probably be the most valuable of these, followed by the BV1 (by far?).