ok novice, I'll give it a shot...
never heard the nht's you mention, but with different drivers in them, even looking up the original specs would be pointless, they're different now. Speakers are designed as a sum of drivers, enclosure, and crossover, and changing one can throw it all off.
low budget... that's not helpful in this hobby... but what exactly do you NOT like about what you already have?
for 80% music, what's the need for a center channel? just use 2 channel. what's the sub? what's the receiver? room size? listening habits?
Lava subs? never heard them. Can't even compare without knowing what you have now, and what you want from any potential 'upgrade'. It may be a sideways step with no benefit except a thinner wallet.
Good receiver? Lots.
4 year degree needed? NO. My 14 year old has read and understands every manual for every piece of gear I have... been great including him in the process... and it is an ongoing process.
Carver amp ? really? big $$ thrown at an unexplained problem there. I can tell you, it wont help with sound quality at low volumes. It may give you more volume at the top end, but living in an apartment? why?
I really don't mean to offend, but you haven't given nearly enough info for anyone to make any recommendations, esp. since you're unclear in your goals. Methinks that's why you haven't gotten any replies. I'd say reading a few hundred more posts is the first step. (man that sounds harsh, and it really isn't intended to, but you've given us so little to go on).
And I'll be the first to say there are many, MANY more knowledgeable peeps here, but if noone else will say it..... your question is un-answerable as posted.
help us help you, please
mods, can we have a 'dislike' button ?