Hi Toid,
I like your 21″ sub woofers a lot. My experience is that closed boxes (with the right elements of course) sound better than vented box. The sound is rappier and the deepest bass doesn’t come one half a beat behind (check Group delay in Winisd). One thing I’m wondering is if the UM18 performs slightly better than the SB 21″ as it has a much larger Xmax? The pump volume becomes larger. I get higher db at 20 Hz according to WinIsd. Are there other reasons why sb 21 is better? Maybe we save power from the amplifier when we can dsp the higher frequencies down a little with the more efficient SB 21″?
/Robban
