I've just completed changing my head unit and adding an amp. I bought a SC901 online; since it was a little blemished, I traded it in at the dealer for $200 and got a "NEW" one with warranty(total cost $475). Then I had to get the wiring harness and amp.(the radio won't work w/out one). I lucked out, in that a tech had an amp from a '91 Bertone laying around the shop and said he would sell me for cheap if the wiring connected to it. Lo & behold, it did! Now the new 850 amp would have been about $250, so I told him i'll only pay $200. When the parts guy punched in the part # for the amp, it was listed at $500+. I figured I got a fair price since it was used. I connect the wires to the radio & amp, and I was in audio heaven. I believe the output of the amp is 50x4; coupled with the independent sub I had installed before, the system sounds ALMOST like the C70. The features on the radio is outstanding(i.e 89.3 is now WQHT 89.3FM). anyway, I also like the custom EQ or you could select the type of car the system is in(there is no 850, so I choose V70 or C70). Not to mention 3CD changer at you finger-tips and type of program selectors.
Total cost for everything is about $900-1000(including the sub install). I had previously installed Infinity Kappa speakers in the doors, so all told it's more like $1300. I believe my tax refund was well spend!

P.S-See the ads. section for info on my SC815 up for sale!!