REL subwoofers have a reputation as a good match with Spendor and Harbeth speakers. Looks like there are a couple of units in your price range listed in the Audiogon classifieds.
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You might want to look into a Velodyne SMS-1 ($450) or SVS/Audyssey ($750) sub controller. My guess is that, together with your existing sub, this will provide the biggest incremental bang for your (half kilo to kilo) buck.
PS - If you search these threads for SMS-1, you'll see that I've written pretty extensively about this device.