Can you have multiple subs plugged into a 7.1 receiver?
Yes, though it usually is best if they are all the same model, and if you like a lot of bass, then it is best if they can each play loudly on their own.
I think you should think about what, exactly, you feel is missing with your system. If it is deficient in bass, and you have experimented with corner placement of your sub (which typically increases perceived output, though not necessarily providing the flattest bass response), then spend the money on subwoofers. If you are satisfied with the bass, if it plays loud enough for you, then look for either changing the room acoustics or the speaker placement or the speakers or some combination of these things. (If it does not play loud enough for you, it might be that you should replace either the amplifier or the speakers or both.)
Room issues for most people typically involve having too much reflected sound, and so things that deaden the room are usually called for, particularly on at least one of opposing sides (thus, carpet on the floor and the ceiling bare is fine; curtains on one side of the room but not the opposite, etc.). Generally speaking, any furniture or decoration helps, as they tend to break up the large areas of flat parallel opposing surfaces. If there is an echo in your room, kind of like the way a typical empty room sounds, then putting stuff in your room is likely to help with the acoustics.
As others have noted, a CD player will sound different from a turntable, and you may not like the added distortion and frequency response variations of turntables no matter what you do. In the case of improving the sound of your record collection, one can spend quite a lot on turntables and such; analog sources can sound very dramatically different from each other. From my experience with turntables in the past (I no longer use one), I found that it is better to have a really good turntable and a mediocre cartridge than a mediocre turntable and a good cartridge. Obviously, it is better to have both top notch, but I would not bother with a particularly good cartridge on a turntable that was not really good. (That, by the way, is not a comment on your specific turntable; it is a general remark. I have no experience with your particular turntable.)
