I'm currently using to Revel Sub 30s with excellent results in a two channel system. Look up Adidadi under member look up he usually can give you great deals and service on Revel products. Also, doesn't Avalon make subs, they might sound the best with your Avalons you have. However I've heard such great remarks about the Rel subs, that I'd probably just hold out for a matching Rel or maybe even one of their better (? I don't know where your model fits into their lineup.)
While two subs are sound much better IMHO, they are definitely more difficult to set up and get right. I hired a pro to do mine after experimenting on my own for 3 months and have never looked back. You might want to buy a second REL from a dealer (I know the company was recently sold to Sumiko, but don't know if they brought out new models, I think they did.) At any rate you might get a good deal from your local dealer, which I understand REL/Sumiko used to insist the dealer had to set up. This might solve two problems at once.
I bet you have a dynamic sounding system with the KSA 300. I have a friend that has a KSA 250 and his system always sounds incredible. You should post your system so we can all take a look at it. By the way he uses all Audioquest with his Gryphon/Krell/Theta/Legacy system and it sounds incredible.
While two subs are sound much better IMHO, they are definitely more difficult to set up and get right. I hired a pro to do mine after experimenting on my own for 3 months and have never looked back. You might want to buy a second REL from a dealer (I know the company was recently sold to Sumiko, but don't know if they brought out new models, I think they did.) At any rate you might get a good deal from your local dealer, which I understand REL/Sumiko used to insist the dealer had to set up. This might solve two problems at once.
I bet you have a dynamic sounding system with the KSA 300. I have a friend that has a KSA 250 and his system always sounds incredible. You should post your system so we can all take a look at it. By the way he uses all Audioquest with his Gryphon/Krell/Theta/Legacy system and it sounds incredible.