@butche34 I doubt you’ll necessarily get significantly more warmth out of a vintage amp. Most amps just sound "neutral"-ish. Intentionally warm amps aren’t really thing. You want the amp to get out of the way. Reviewers compare and contrast different amps, so differences are exaggerated. You could add a tube pre-amp or play with EQ presets to add "warmth". I don’t have much experience with that.
I’m sort of an audio newbie as well. I dipped my toes with a overpriced poorly built buzzing tube amplifier, a dirt cheap 10W Tripath class D mini amp and a dirt cheap vintage Continental Edison PA 9109. They all sound pretty similar with my speakers (Closer Acoustics Ogy).
The Marantz PM6005 is simply great and not worth upgrading.
Re: vintage amps. They’re so affordable if you know where to look: thrift stores, local classfields, flea markets, garage sales etc... Be warned that the condition will be rather ’poor’: dusty circuit boards and you’ll have to replace a bunch of transistors. So contact a local hi-fi repair shop and they’ll definitely guide you in the right direction or fix your amp like new for $100 or less. Or you could buy an old refurbished amp from a pro for $200 with a 3 month warranty.
I bought a dusty Continental Edison PA9109 (20W AB amplifier) for 40€ from a local ad. It’s a pretty unremarkable amp on paper. The amp stopped working after 1 month. I went to a local Hi-Fi repair shop and got my unit refurbished for 84€. The shop owner told me that he replaced at least half of the transistors and he cleaned the inside really well. I would do it all over again. 124€ for an amp is a steal. If it stops working in a decade or two I’ll just get it fixed again.
Unfortunately, the internet hyped up the good overbuilt, over-performing models from Pioneer, Sansui, Harmon Kardon, B&O, Marantz, Luxman etc... So you’ll be stuck with run-of-the-mill brands like Kenwood, Technics etc... in the < $200 range. Or spend $600~$800 on the ’good’ stuff.
And I doubt that there will be a significant difference with your Marantz PM6005.