Tennis, there's some good stuff about your inquiry on the McIntosh forum at audiokarma.com.
I am not sure if you would consider these old enough to be NOS. I have been using these 6550's in my Prima Luna Prologue 2 integrated and Prologue 5 power amplifiers. Prefer these to KT88's and EL34's.
SED Winged C 6550 tubes.
If the sound you're seeking in your preamp is what you want from your amps, the only 6550 I'd suggest would be NOS Tung-Sols. And other than that, I'd suggest getting NOS Gold Lion/Gold Monarch/GEC KT88s. As a general rule, the 6550s will have tighter bass, but a leaner midrange, than the KT88s; not sure you'd like that sound, from your other posts. The Svetlanas that Rich suggests are a current 6550 that appraoches the sound of the old Tung Sols, and may well be richer in the mids than current KT88s, I haven't really listened to the new ones.
FWIW, regarding Reprince's comments and Tvads concurrence, in my experience the 6550s (SED's especially)is similar except I would describe it a little differently. I consider the 6550 to have a more linear sound with a softer high end than the KT88 which I have found to have, in comparison, a bit of bloom in the upper bass and some 'sparkle' in the upper mids and highs. The KT90 and the 6550 are somewhat similar except the KT90 is not short of high end energy. Which of these tubes sound best greatly depends on the amps they are used in, and, obviously, your system and tastes. I use both of these tube types in two different amps and neither tube was optimum when used in the other amp. Called system synergy!
One caveat for those considering 6550's who are seeking 'warmth'. You won't find it in 6550's (or KT90's). EL34's and 6l6GC's (if you can use them) are a good alternative for mid-range, and the 6l6GC has a softer upper-mid range highs. Quite a nice tube actually, but neither of thise have outstnding bass.
Mt T if you can do without the bass of a 6550 the 6L6 has tone you should love. The Mc probably will not use it though.
My apologies as I know that this started out as a Mac thread. My exeriences are with two tube amps, the Prima Luna Prologue 2 &5. I was very disappointed with the EL34's in the Prologue 2. I felt that the stock KT88's just sounded better ... more alive and 3D, if you will. I tried the 6550's (I think on your recommendation) and was impressed with the way they "smoothed out" some of the harshness that I was hearing. I use only digital sources. There were CD's like Placido Domingo's "Verdi Aria and Duets" (EMI) that could get downright annoying when the orchestral fanfare kicked in. The 6550's smoothed things out, without muddying up the detail. Rest of the system: Rega Apollo CD player and Acoustic Research 302 speakers.
System synergy ... absolutely.
One additional note. I added the 6550's almost a year ago and then switched components between two of my systems and sold off what I wasn't using anymore. Since then I am feeling a sense of satisfaction, better said calm, with my systems, for the first time in a long time. To do much better, I would have to drop a lot more cash.