Tubeperson, I'll offer a perspective on your last question based on my experience as a previous long term owner of Marantz 9s (20+ years).
As much as I loved those amplifiers (particularly when run in triode), imo, they can be competitive with the best designs of today only with parts upgrades; in purely stock parts condition, the increased quality of capacitors, resistors and other components available today simply move current high-end tube equipment past these classic beauties. Of course, doing any parts upgrades would materially reduce the value of a "museum quality" pair. In my case, I made a choice to upgrade a number of critical capacitors and resistors, and to bypass the bass filter and volume control (no circuit change, just parts upgrade as Saul Marantz might have done himself were the parts available to him). There was a material increase in resolution and transparency with those modifications.
But even with those modifications, I've found several contemporary amplifiers to be more truthful in their musical accuracy than my long cherished Marantz 9 amps. For example, the current Conrad Johnson Premier 140 or the older Premier 11XS (using EL34s in triode). And, for my musical priorities, I find my current pair of Atma-Sphere MA-2 OTLs better suit my needs in terms of neutrality, speed, resolution, dynamic contrasts, and extension at both the top and bottom of the frequency range. For my listening priorities, I simply could not return to the Marantz 9s as my primary amplifier, much as I value and admire it.
But, a classic pair of Marantz 9s are amplifiers to be cherished. With excellent, well selected vintage tubes, they still will charm the socks off most other amplifiers and they are beautiful to listen with. I do think they are well worth pairing with some of the best tube preamps available today, because the best preamplification will still improve what your Marantz 9s are able to deliver. Best wishes in your continuing enjoyment of these classic treasures.