I have heard ls3/5s with a sub (Actually the harbeth sub, not REL) and while it was good I'd probably get the compact 7. My reasoning is that the C7 is an all-round better speaker, and that at a later date you could add a sub if you get the desire and the funds. If you wanted to upgrade the P3/sub setup you'd have to sell one or other (probably the P3s) and each time you sell you lose money. Best to get the best you can and add.
