One Sub or Two?

2

I think I'd wait and see how they sound in your room.  I wouldn't expect a speaker like an 801 D4 to need much help for two channel. I think you'd need really good subs to gain anything. It's interesting that REL's website doesn't have a recommendation. They recommend their model 31 for the 800 D3. It's a $7500 sub. I'm not saying you need to buy a REL, but you need a really good sub. 

How big is your room? How much do you want to spend? 

Two subs for music. 

I went with F12SE Rythmiks because of the ability to dial them in variably (rather than 0 or 180, as the case is with REL subs) to a pretty tricky room. I got a pair. Love them. I have an older REL, too, and it is my third sub. The subs are coordinated using MiniDSP (link is about subwoofer integration).

Again, REL has a huge fan-boy contingent but not many models have variable phase. REL has 0/180, mostly.

A simple 0/180 switch allows for a complete reversal of the subwoofer’s polarity, effectively inverting the signal. But variable phase control on a subwoofer allows you to fine-tune the bass output to blend seamlessly with your main speakers, maximizing bass response and avoiding cancellations at the crossover frequency.

Two REL's, which ever models you can afford.

I have several systems and they all have two REL subs and I'm very happy with them.

Information about the size of your room would be helpful.  Based on the speakers and the HT / 2 channel mix, assuming a larger room, more is better.  No need to stop at two.  Highly recommend Rythmik servo subs over REL.