Will McIntosh C2600 tube preamp solve my problem?

A dac is a digital to analog converter.  If records are your only or primary source you likely don’t need to pay for that feature.  However, if you are using an old CD player and it has a digital output you may find a modern dac offers an improvement.

Your Rotel RB 1590 is 350 Watts per channel which is more than enough power for a large speaker like your 802 D3’s.

 The RB 1590 is not specified as a dual differential amp so the XLR inputs are likely just a convenience. So a preamp with RCA inputs and outputs will work.  However, there is one problem.  The input impedance via RCA is a very low 12k ohms while the XLR is 100K.  Most tube preamps will be compromised seeing 12K ohms.  So that means a preamp with XLR output connections would likely offer the best performance.  Preamps so configured are more expensive plus you will need a quality set of XLR cables to connect to your RB 1590.

Does this help or add to the confusion?

Don't understand the question of "what is my source component".  Can you be more specific?  Sorry for my lack of knowledge.

Don't understand the discussion of "XLR output connections", but yes, it does help because I can research what you mean and ask my local dealers.

“Source” is analog like a record player, or digital like a CD player.

RCA connectors are what you are familiar with like what is on your old preamp, just one hole in the middle with a barrel for the cable to grip on the outside.

XLR’s have three pin holes and appear completely different than an RCA connection.  The connecting cables are completely different in appearance.

Thanks for feedback.  Looks like I have some more homework to do.