why not exchange your 4 ohm subs for 8 ohm subs? (I'm assuming the reference subs are made in 8 ohm form...)

If you use (2) 8 ohm subs, you can bridge the amp and run the two subs in parrallel. The total load on the amp would then be 2 ohms - which is right where you want to be.

 

I have the JBL BP600.1 mono Class D subwoofer amp, running 2 10" Audiobahn dual 4 ohm voice coil woofers and works great. The 2 woofers have dual 4 ohm voice coils meaning each woofer's impedance is 2 ohms, and when connected in parallel forma 1 ohm load for the amp. The JBL is rated to handle a 1 ohm load and does so perfectly, and output is rated at 600watts!

And the best part is, I only paid $185.00 for the amp on Ebay!

Just my 2 cents....