Do I REALLY need preamp and what do I upgrade the amp too?

Hey everyone-Let me share what I have and then get to the question:

Rotel RC-1590MKII

Rotel RB-1582 MkII

Gustaed DAC-X26 Pro

MAC Mini M2

B&W 805 S2 on 30” stands.

2x Rel T/5x subs using the oututs of the Rotel RB-1582 MkII

Nordost Blue Heaven Speaker cables bi-wire.

2x AQ Colombia RCA interconnects

5x AQ NRG-Z3AQ Power Cables

Core 1000 Power Conditioner

The room is 12x14 with area rugs.

Large L shaped desk with multiple 32” 4K monitors for work.

8x6 open wall is treated.

Ceiling is vaulted.

I listen 100% via Qobuz with Audirvana; all types of music.

Recently I bought an outboard DAC to see how it compares to the internal DAC of the 1590 and I was blown away at the improvement over the DAC built into the Rotel. 

Just today I thought why I would need a preamp if all I use is a the Gustard with variable outputs.  Why not go direct to the amp?

WOW OMFG!!  The difference was spectacular to say the least.  Not saying at all Rotel is a 2^nd class, its not, I have massively enjoyed it over the last year.  It allows Roon and Sonos interoperability (Port) which I use often, but I am willing to make a sacrifice.

I have been a silent member for a couple years now.  So, given all the data above, if I sold/traded the Rotel pieces for a sinngle upgraded amp; assume I can get $2K USD out of what I have (concretive) and added $2K to the transaction what would the community recommended? 

I do like to shale the wall and believe in leaving SS electronics on all the time, so I guess that rules out tubes. 

Thanks in advance I am looking forward to the discussions.