Inna, I'm not sure which way to go yet, but it seems to me that adding bass is an obvious path to follow as a next step. The RELs have been used before with LS3/5a, but it's a little unclear whether owners of these speakers wholeheartedly agree that the addition of a sub is a positive thing. Anyone who has fiddled with LS3/5a and different types of subs, please share your experiences.
Without going as far as phono amp/MC/new deck, maybe a better cartridge and a sub would max out my system well enough. Stopping at diminishing returns sounds like a good idea. Already the vinyl and recording qualities show up very transparently on this system.