For what your budget is I don’t think you’re going to find anything new or used that’s going to crush your 7B’s; now if you’re just wanting a different sound altogether that’s a different thing. Your post started out looking for a speaker with a little more bottom end and the fact is your current speakers meet or exceed what your room can produce based on the dimensions you gave - it’s purely physics. There’s a lot of factors that could be why your room can’t support the low end it’s capable of such as are you on a concrete slab or a suspended wood floor system, how are your walls constructed which there’s many variables there and are you on an exterior wall with a large window with no window treatment such as heavy fabric curtains? Do you have any sort of room treatment and is your listening seat up against the wall? Are your speakers on the long or short wall, and if on the long or wider wall have you tried them on the narrow wall? Just hate to see you sale what you have and spend the money and not get what you’re looking for. I think you’re going to discover your room has a lot more to do with this than your speakers.
If you like your 7B’s I can recommend ProAc as a line to look at. I’ve listened to many speakers over the years as I’m fortunate to live by two great stores that cover or have covered about everything out there and I’ve heard many. Today my favorite is the B&W 800D3 which although up there is a speaker that delivers unlike many I have heard at two or three times their price.
I’ll close by saying I was floored by how much my 7B’s improved when I added dedicated circuits to my system which made it obvious my amp wasn’t delivering what it was capable of to my speakers; it wasn’t my speakers but the amp that was starving. Just food for thought if you’re not running dedicated circuits to your equipment. I’ve spent a lot of time tweaking my system with cables and etc. and I’m amazed how revealing my 7B’s are with subtle tweaks. The more time I spend with them, I’m confident they’re capable of better than what I’m feeding them.
Whatever you you end up doing, if you can keep your 7B’s before selling to compare.
Goid luck and keep us posted.