I have the Plinius M-16 with phone stage in one of my systems and like the preamp very much. Im basically using that preamp because I really like the phone section, the preamp itself is very nice also and as you know, built like a tank. I have not heard the M-8, but if you are looking to save some space by not having to add an additional phone stage I think you would be happy with the M-16. Im sorry I could not tell you the strengths and weaknesses of the M-16 v. M-8.
