I would also say that the Border Patrol leans pleasantly warm, but I wouldn't claim it dark or having a particularly black background. I have a xDuoo MT-603 coming in the mail today or tomorrow that I was planning in running from my BP and I have a pair of dark and warm Sylvanias, I could let you know how that goes if you don't mind me cheating.
I think the xDuoo MT-603 was a pretty ignored product because it got marketed as a preamplifier, but it's actually a tube buffer with no amplification stage. If it's good, it really has no competition because the only other recent and similar product (that I know of) is the Ifi Micro iTube Tube Buffer and that got discontinued a while back.
If you ask me, it looks like the right cheat code and I don't mind cheating