Audio research

 There were a few different versions of the SP9. It should say on the back if it is a mk2 or later. If you email ARC with the serial number, they can tell you what version you have and when it was made. Around 1990 or so ARC stopped putting mounting holes in the face plate.

To help know which version, there are photos of the internals here at the ARC database. http://www.arcdb.ws/SP9/SP9.html

Just don’t place any stock on the ARC supplied sound quality descriptions. As can be expected, each one is at the same time the best thing ever and much better than its predecessor. 

Keep in mind the SP9 is not the worlds best sounding Audio research preamp and that was not part of the op's question but the newer Audio research LS1 has the same tube linestage as the SP9 but without the phono stage. There are many people who liked the LS1 and I owned one and eventually moved up to the LS 2 MKII B with remote which I liked even better. Currently I no longer own either one  and have moved up the audio chain.