I sell loads of underseat ones and they work really well. Pioneer ts-wx110 is quite small as it uses a single 6x9" woofer. Focal ibus20 uses an 8". Ts-wx77a certainly isn't underseat ! Look at the dimensions - it's nearly a metre long!!!
I've fitted a quite a lot of the focal with really good results, I've also done some of the Boss versions along with the Rainbow one too (looks like a copy of a Focal).
New small Alpine one works quite well (SWE something something, can't remember full part number as I've switched off from work mode at 11pm!)
If you don't mind something small in the boot then look at the new Hertz ES F20 and F25 slim boxes (need an amplifier too), or the new Jl Audio CP108 subwoofer box which uses a JL Audio 8w3 in a special slot port design box ( just listen to one if you can find one to demo - they sound amazing, but no built in amp and not cheap) (if you can find the JL Sales Managers Blue VW Scirocco then it has one fitted)
Also the Rockford Prime and Punch slim range in Rockfords own boxes work really well
Hope this helps
Andy