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.

What ever you buy you need speaker high level inputs.

The REL has a high level input.  From your integrated speakers output, to the high level REL inputs via a SpeakOn cable with one end bare. The bare wire goes to you speaker terminals on the amp, as per instructions, Red right, positive, Yellow left, positive, black left or right Negative. 

Then out of your bass bins, high level outputs to you speakers, binding post..

Happy hunting

Regards

Another vote for REL. Through High Level Input, the sub integration with your main speakers is straight forward and seamless. There are plenty of YouTube video from REL on the high level connectivity.  

https://rel.net/tube-not-tube/