I would not do any upgrades to the current preamp and amp. It’s unlikely to make a significant difference and you will never recoup that investment.
Best upgrade path in my opinion would be speakers. Something like the GoldenEar Triton 3 used. You would solve you brightness and bass issues.
If you don’t want to part with your current speakers consider adding a subwoofer which would fill in the missing bass and tame the treble since it most likely sticks out due to lack of bottom end extension. The challenge with adding a sub is properly integrating it.
I wouldn’t worry about cables. Monster cables are not great but they’re not your problem.
Also consider adding some room treatment. GIK Acoustics make good panels and they’re not expensive. You can upgrade speakers and get few panels and stay within your $2,000 budget.
@ghdprentice recommendation of tube preamp is also a good approach. You can pick up a used Rogue RP-1 preamp for around $1,000. Combine that with a sub as it has 2 pairs of RCA outs one of which you can use to drive the sub
Good luck!