Set-up with two subs and an integrated amp

REL subs use the SPKON connection which utilizes your amps outputs. No RCA sub-out needed.

Unfortunately there is no good solution if there are no line level outputs. You are left only with subs that have speaker level inputs which leaves you without a high pass filter a situation totally unacceptable. I would wait on subs until you have the appropriate electronics.  

+1 on the REL solution. REL integrates perfectly with the SPKON conection. If you get a preamp with an RCA sub-out later, REL can do that too. Wireless optional.