Anybody upgrade their SVS PC 12 Ultra and/or Plus subs?

They want a small fortune for their drop in replacements

Sangway has 'similar' plate amps for a ton less

They also offer them in 120v 

2 @ $245 delivered!  SVS wants $495 plus for ONE
No point upgrading a sub until you have at least 4 of them.

I know Scaena Loudspeakers uses 4, but they are stacked 2 x 2

I tried using one but 2 was that much better. Would 4 be? I likely will never know

I've got two of the SB13 ultras and they seem like plenty with my Wilson WPs.