Starting afresh with a Linn Sondek LP12...

Not all components are good to buy vintage. A vintage tube power amp may work OK for you, but there's no way that I would buy a vintage preamp.

Annika, welcome to Audiogon!

The choice of a suitable amplifier will of course be highly dependent on your choice of speakers. Since you appear to be gravitating toward relatively low powered tube amplification, it would probably make sense to choose speakers having fairly high efficiency, perhaps in the 95 to 100 db/1 watt/1 meter area or thereabouts. Although this thread is a few years old, reading through it will probably yield a number of possibilities to look into, albeit mostly modern ones.

Given that you appear to be envisioning a vintage preamp that includes a built-in phono stage, the choice will depend on what kind of cartridge is installed in the LP12, and especially on whether it is a low output moving coil type (rated to provide just a fraction of a millivolt output under the standard test conditions), or if it is a moving magnet or high output moving coil type (rated to provide a few millivolts output under those conditions). Let us know what model cartridge it is.

I owned a Marantz 7 (also known as a 7C) for a while about 20 years ago, which I used in conjunction with a moving magnet cartridge. It was an excellent performer, and I wouldn't hesitate to recommend its use in conjunction with high output cartridges. PROVIDED, however, that it is in top condition and/or well restored. And, ideally, if you would enjoy a bit of experimenting with various makes and vintages of the 12AX7 tubes it uses, to optimize its sonics.

Also, be aware that VAC (Valve Amplification Company), the highly regarded American manufacturer of tube amplifiers and preamps, during the 1990's produced a Marantz 7C reissue. That could very well be an excellent sounding but easier to find and less expensive alternative to the 50+ year old original.

Another possibility that may be worth seeking out would be a good example of one of the early solid state Mark Levinson preamplifiers from the 1970's or early 1980's (not later), provided it is suitably configured for the cartridge that you have. I still use the phono section of a Mark Levinson ML-1 preamp as my phono stage, accessed via its tape out connections.

Finally, some of the Audio Research and Conrad Johnson tube preamps from the 1970's and 1980's are well worth considering.

Regarding your question about AC step-down transformers, I don't know the answer. I recall that there have been a number of threads here in the past on similar questions, which you can find via a search. I'm not sure, though, that the answers I have seen provided in some of those threads are necessarily authoritative.

Best of luck. Regards,
-- Al

With the Sondek as source. Naim amps of a similar vintage ( which can still be serviced) along with either Linn or Naim speakers ( Sara or SBL's) etc that can be put against back wall would make for a classic 'flat earth' system great for classic rock.