I have the same Motion 40/30 setup in Dark Cherry and I have been looking for a matching sub as well ... HSU offer 3 subs and a mid-bass module in rosenut that seem to be the closest match in my eyes (and I've spent considerable time looking) .
I called my local audio salesman who sells both MartinLogan and Monitor Audio, wondering if the MA rosemah finish is a close match and he suggested that it is not.
Yes indeed, the ML dark cherry finish is beautiful ... When at the store ready to order the Motion 40s, my wife and I settled on the piano black because of the cost (another element of the WAF) ... When we got home and unpacked the 40s, lo and behold, they were dark cherry. We ended up splitting the extra cost with the audio store. I've subsequently added a pair of LX16s and the Motion 30 - all in the dark cherry as well.
Hopefully in a year or two I'll be able to afford the HSU VTF-3 MK4 ($898 shipped) or the ULS-15 ($1474) and my son will get my ML Dynamo 700.
MartinLogan Motion 40s / 30 / LX16s
Marantz SR5007 / UD5007
exercise room: MartinLogan LX16s / Dynamo700