Adding a sub woofer?

@hilde45, Duke,

Take this with a grain of salt. Varies by the listening room.  While I prefer stereo subs, it’s all room dependent IMO. Phase and frequency cancelation at varying lower frequencies "can" occur with two, particularly in uneven odd shaped rooms -or- in situations where you are not able to place a bunch of room treatments or not enough phase adjustment in the sub x-over itself. I try not to advise on this as its mostly due to an environment-room issue.    

I still run stereo subs now, and often times leave one subwoofer powered off based on my seating position [in my case, in a particular room]. If I run the same setup in a different room, with even side walls, no issue, dual subs works great. Again, more of a room problem and no guarantee until you try it either way, YMMV.  

I have a very good tube untegrsted in Ayon 
and a very solid Yamaha 2100 both have active preamplifier sections , SVS, or crutchfield sells a resistor adaptor that connects directly to speaker outs on the amplifier , it is more accurate 
and better Bass control 
if you sub has speaker inputs, like rel  has it’s a very good setup .
I use this which it has a + and - per channel ,you use awg 16 wire I use Neotech and crimp bananas to one end to go into the speakers ,the other side plugs into the adaptor board and there 
you plug in rca sub cables into the board ,the other side into the sub input , it’s sounds like a lot of work ,but it’s not and only
$30 I Velcro to the back of the audio stand.

@hilde45 and @decooney , I thank you both very much. 

Duke

Another brand of sub you might want to look into is HSU VTF-2 MK5 or the VTF-3 MK5 HP

Adding a sub(s) is one of the most challenging things to get the sound response you want. Ideally, what millercarbon suggests is the most sensible approach to this challenge. The approach to adding multiple subs (sometimes referred to as "swarm") is ideal.
However if you are not ready to get multiple subs you can look at the various wiring techniques mentioned here. Or, you can look into integrating a sub(s) by using an active crossover with bass management capability. Those crossovers can be from reasonable in price to very expensive. My solution was getting an Outlaw ICBM crossover. Original retail was $250. Don't believe it's still in production but can be found in the used market. A crossover of this type, with sub out connection, can be connected between the pre and power amps to perform bass management as well as adjusting crossover frequency to the mains. Not the supreme setup but can make integration of a sub much easier. That's the most important thing rather that how you wire up the sub.