If you like your vtf3.4, you are going to like the FV15hp a lot. I upgraded to the FV15hp from HSU VTF2.3 and the difference is substantial. Like you, I love the HSU vtf2.3 but wanted to upgrade to the vtf15h but when I went and auditioned the dual vtf15h's at their head quarter in Anaheim, CA, they did not impress me as I expected. The hunt continued and after reading threads after threads, reviews after reviews, and all the GTGs here, I decided with the fv15hp (550W version which is no longer available), and it is a hell of a sub (deep, loud and articulate). The XV30 was not available when I upgraded but from what I have read here, it is also a great sub.