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There are so many ways to go on this. Budget-wise, the Adcom GFA555 (or later versions of the same 200 watt amp) work well. I am using one with my Kinergetics SW800 subs and it works fine. Also, the larger Parasound's are good sub amps for the money -- HCA2200 or the HCA3500 (350 watts). I used a McCormack DNA1 Deluxe with great results. I would think the 225 would kick ass. The key is to get an amp that doubles (or comes very close to it) into 4ohm loads. As I said, there are many good options. With my subs there is a definite difference in sound when using different quality amps (they are forward firing and a line source down to 200 cycles). I do not know if the Hsu's react the same way. Bryston 7B's are my planned upgrade when I get to that point. If you have the scratch, these would be a great way to go. I have heard that the Marchand x-over is very good but have no first hand experience with it.
jim's adwice is good. (i know - he also likes alfas!) i use two adcom gfa555's to drive my 4 ohm vmps larger subs. i was running 'em bridged yust fine, but due to all the recent talk of bridging halving the load the amp sees, & halving the current it outputs, i tried using yust one channel of each amp in stereo-mode. well, the bass *is* yust a mite tighter. w/the subs being 95db/1w/1m, 325wpc is adequate! :>)
re: x-overs i use the marchand xm-9, & it *is* an excelent unit - wery transparent, & *wery* flexible. while i haven't a-b'd it w/a well-known & respected unit such as the bryston 10b, more than one person familiar w/both units has told me the marchand is more transparent. at half the price of the bryston, this is a good ting! :>)