Filter topology of 6581

[mackeSID]

Hello!
I've seen the amazing stuff that can be done with later revisions of the chip (6582 & 8580), with feeding the output signal back into the input causing the filter to act like a 4-pole configuration.

I've done some research on this and I've found the 6581 I have doesn't seem to work the same way.
After activating the filter in function of my sammichSID, I've used an analog DJ mixer to mess around with the signal before feeding it back to the input by using the headphone output plugged into the input on the Sammich.

What I've found is that the input can be saturated which creates huge amounts of bass, as well as a nice n round clipping effect, but no crazy resonant stuff like the ones provided at this page:
http://www.bigmech.com/misc/c64mods/feedbackloop.html

According to the 6581 datasheet, the input is supposed to be around 300mV, so I've tried staying just below that when feeding back the signal.
Also, the only reference to the filter is that it uses integrating capacitors.
Am I correct in assuming this means it's an OP based filter with a software-adjustable resistive component to control the cutoff frequency?

I'd really like to understand how one could increase the filter order, as I use my sammich alot for TB303-ish basslines. ^_^

Have it good
/M