Mixed Brand Pre-Amp Power Amp Pairings that Work

I have a Luxman CL38u se tube preamp and a Bryston 3B Cube amp w/a pair of Aerial 5T’s. Very happy w/the combo.

I'm running a Musical Fidelity MS8  preamp with a Parasound Halo JC5 with fantastic results.

The obvious answer is to read reviews and demo as much equipment possible and identify a pre and amp (or integrated) that match with your sonic preferences and the rest of your system. Another way to go and which is how I ended up building my system is to pick a very neutral and uncolored preamp (Bryston BP-6) and paired it with my McCormack amp that has sound characteristics that really appeal to me and that the Bryston just let shine through unadulterated. Point is there’s more than one way to go hear and there’s no one right answer.  No question an integrated will be easier and likely cheaper way to go unless you’re purchasing megabuck components or want the flexibility to mix-n-match brands and sound characteristics. Hope this helps, and best of luck.

I have a Luxman C-900u driving a Sugden FBA-800 and the pairing is excellent!

Let's face it: a whole lot more combinations are going to work just fine, compared to those that won't.  The one technical issue has already been mentioned: impedance matching.  Beyond that, I suppose you could end up with a bright preamp and a bright amp that together were too much, and likewise a syrupy preamp with a syrupy amp. But pretty much the world is your oyster. I'd wager most same-brand preamp+amp combos are because the owner particularly likes how that brand sounds, rather than any other concern.  Similar observations apply to the "full loom" issue.