[I listen to NO vinyl- is the phono port dead to me or can it be utilized for something else (same Q applies to the tape, et al, ports...)?]
You might want to try vinyl someday, and all you would need is a turntable with a magnetic cartridge and a source for new/used vinyl, the Marantz was designed with the needed phono stage built in to the receiver (as were all receivers and integrated amplifiers before late 1980s or maybe 1990/91.
You can't use the phono input for any other source as the expected signal is only 5 millivolt, or about 50-100X lower than the typical signal of CD player or tape deck or computer sound card. These sources would overload the phono input, distort badly, and possibly damage components in the phono stage. Also, the phono stage is equalized using the inverse of the RIAA response curve that is found on all vinyl records since the late 1950s. When you play the record, the phono stage compensates by boosting the bass and cutting back the treble restoring flat response. You can use any of these high-level interchangeably for CD, computer, SACD, TV sound, etc.
[So if I'm NOT using 600s (yet ), will this actually result in a decrease in SQ compared to my A900s, even if still run through my HA-1?]
It's not that the 2252 can't drive the A900 headphones, it can do so just fine, while have mountains of reserve power for this application. However, because the headphone output of the Marantz is loaded down with a network of resistors, the resulting output impedance is between 100 ohms and 200 ohms. The A900 are rated at 32 ohms, so the frequency response of the phones may be affected such that the upper frequencies will be rolled off. It still may sound very good, but you won't know until you try.
[I love the vintage look and the idea of having a tuner on hand. If I was getting this for $230 shipped, would it be a good deal?]
Getting one practically for free would be a good deal. When I see these Marantz units being given away for $50 - $100 they gnaw at my willpower to resist the temptation to pick up another example as a spare, for a friend, or because it is just too good a deal to pass up.
Unless you are a serious collector who would only consider buying a piece of Marantz in mint condition, or if you had one way back when and are trying to rebuild your first good system, I would do more research and wait for a better deal. The best sounding Marantz units, for headphone use, are not the more powerful models. I would concentrate on the low to medium powered models (rated at 10 to 35, or perhaps 40 watts). These may actually sound better as headphone amplifiers and will cost a fraction of what the 2252 you found will cost you.
When you start to approach $250 or higher, there are better choices for headphone amplifiers including a used MG Head OTL, and vintage tube receivers and amplifiers from Fisher and Scott.
When the price starts to approach $300 or $400, then a good modern solid state headphone amplifier such as the Corda, Gillmore, Grado, or Rega is within this price range. These modern amplifiers lack the phono stage, multiple inputs, tone/balance controls, and the FM tuner of the Marantz, but they have a much smaller footprint than the Marantz equipment and depending on the headphones, may provide better sound.
