Hi
I had a pair of M3s on stands and enjoyed the warm, clear sound in my study, therefore quite a small room. However, they had been completely refurbished and the cabinets had been changed to Piano Black lacquer. They sold for approximately £330.00 last year.
I also have a pair of M60s, which are similar but a bigger presentation.
I note that you have a pair of DSP5ks, if they are 18bit, these again in my opinion give a warm, clear sound. The 24bit versions are quite a lot more precise, in particular the treble/tweeter.
Taking into account that these M3s are likely to around 40 years old, unless they have been serviced, the sound will have degraded over time and you should factor that into the price you are prepared to pay.
Hope that helps.
Cheers
Jon