new member here

Just joined to find out some information on a future system. Looking at the Motion 15's for the founts, Motion 8 for the center, and either Motion 4's for the surrounds or some in-ceilings. Looking forward to getting some advice.

Welcome! You've come to the right place. Hopefully, someone with Motions will be able to give you some advice. Be sure to be specific in your questions. It always helps to know the environment (system, room, etc) when asking advice. Also, pictures can help.

Again, welcome : )

As a long time Bose guy, ugh, my whole listening experience has changed in the past two months after looking for a pair of bookshelf speakers for my son ... We weren't zeroed in on martinlogans initially, but when we went to listen to some other brands, we were quite impressed with the martinlogans.
I bought James a pair of Motion 15s, a pair of Motion 40s for myself, a pair of LX16s (last year's version of the 15s ... basically the same speakers), and a dynamo 700. The difference between the MLs and Bose is night and day.
By all means yes, treat yourself to a pair of Motion 15s ... they sound great. They will need, however, to be complimented with a sub, as would any bookshelf speaker. James is very happy with his 15s, and my LX16s sound just as good.
Currently, I'm running a 3.1 set up, with one of the LX16s for a center channel (the other one is idle at the moment ... I'll buy a single 15 or 16 in the near future to complete the 5.1 arrangement.
You may want to consider a Motion 15 as a center channel as well ... they are sold individually in many cases ... and you may be interested in the issue of "lobing", which relates to the common center speaker design ... Three identical speakers in the front of your set up ( in your case Motion 15s ), would be awesome, IMO.
Additionally, the dynamo 700 is all I could ever hope for from a sub ... It's in a different league from the other subs I've owned.
Let us know what you decide and end up with ... good luck : )

I forgot to ask, do you own a sub to go along with the other speakers? If not, I'd recommend a sub before buying surround speakers, and quite possibly before a center channel as well ... 2.1 or 3.1 set ups are quite acceptable, again, IMO.