Strings with black felt mean they are almost certainly actual Cirrus strings. Nickel roundwounds. I really like those strings and they're cheap, too! The local shop where I was buying them in Tallahassee sold them for $15 - 20 a set (depending on whether they were 4, 5, or 6 strings).
The Millennium preamp (aka Peavey Custom Shop preamp) is standard in a Peavey Millennium but only came in Cirrus made in the Peavey Custom Shop. It's the same as the stock preamp except that the Mid knob changes to a stacked knob setup where the top knob is still Mid boost/cut but the bottom knob is a mid-sweep. I.e. it lets you adjust the frequency center that is affected by the Mid boost/cut. I think it lets you adjust that mid center frequency from 200Hz to 2000Hz. But, even if you leave the EQ flat, I still think it improves the sound of the bass a slight bit.
I have had a Peavey G-Bass, G-V, Cirrus 5 and Cirrus 6 that I converted to the Millennium preamp. I had the Cirrus 5 already done when I got my Cirrus 6, so I was even able to do a direct A/B comparison between the 5 w/Mill pre and the 6 w/stock pre. At which point, I ordered another Mill pre to go in my 6.
The reason for the new knobs is that you have to get new knobs for the mid pot on the Mill pre and they don't have a stacked knob set that matches the cosmetics of the stock rubberized Cirrus knobs. So, you could just have 4 rubberized (stock) knobs and one nice knurled metal stacked knob on the mid pot, but it will look nicer to get a complete set of matching knobs with the stacked setup.
Detailed info on the upgrade is here:
http://www.talkbass.com/forum/f38/peavey-millennium-preamp-upgrade-step-step-395853/
There are phone numbers and part numbers on the second page of the thread.