[Hondasucks]

I've heard of first gen legacies getting crap mileage due to a faulty knock sensor, so it probably applies to SVXes as well.. I'd suggest getting the new style knock sensors that have the connector right on the sensor off of a newer EJ motor, along with the enough of the mating connector and wiring to wire them into your car, they should be interchangeable but I've never tried...
Although I think checking your tire pressure (I have mine at 34 PSI F/R) and replacing your plugs (NGK V power) and fuel filter (OEM or good quality aftermarket, NOT Fram) would be where I'd start since those are easier than the knock sensors.

I'm not sure how to test the knock sensors aside from hooking up a timing light and tapping on the block around the sensor with a screwdriver handle and see if the timing retards (Depends on if the ECU is smart enough to ignore knocks that aren't in time with the coils firing, it's old enough that it probably isn't, but it's advanced enough in other areas that it might be)

You can use a timing light to check the timing, but if it's off it's not adjustable, if it's off it means there is a problem (like faulty knock sensors), but I am not sure how to do it on an SVX, nor what the timing it supposed to be... I can look in the factory service manual tomorrow and let you know.