Making a subwoofer to complement Genelec 1029a

[VvvvvV]

Hey everyone, I've just got some genelec 1029a really cheap from a media company, and at the moment the only subwoofer I have is a Celestion s1i that was also really cheap.

at the moment I'm running just one of the monitors using an unbalanced line ( and the only lineout that I could find on the Sherwood) and the manual tells me that the cable neutral should be grounded at the output end in the case of unbalanced lines. My one is grounded at the input of the monitors, is it important, should I stop immediately?

The subwoofer complements the monitors well, and I have run some sweeps of the crossover range and it presents no obvious flaws with good sub placement to adjust phase .

The trouble is that a sine played through the subwoofer sounds like a triangle, really paltry base , and the key presses at the lower end sound more like a portamento than various strait pitches.

Should I gut the subwoofer box and try a nice new driver along with a brand-new amplifier and try various electronics things until it sounds right? The celestion crossover/amplifier has about 100 resistors so I couldn't really improve it, the only thing I can be sure of is where the transformer is.
should I go to an audio shop with the Speaker and the Sub and just try various drivers until one sounds decent?

The sub cabinet is about 60 L/15 gallons down firing with a reflex port just near the driver . What should I do with it?

What would you do on a budget to have some serious smooth fast extension for the genelec? http://www.genelec.com/pdf/OM1029a.pdf

apparently the audio thru going to the subwoofer at 100 dB spl is-23 dbu into 33 kohm load -is this important, what does it mean, isn't it just a line output?


Cheers