I saw the review Carbonyl linked to before I bought my 641's. Which made me scratch my head a little bit , but since I liked the sound of the previous 3 sets of Altecs I had (ACS295, ACS345, and ATP3) I thought I'd give the big boys a shot, especially since I had listened to the Z-560's and was seriously underwhelmed, and the ProMedia's really hurt my ears.
The 2 main criticisms in the review were slightly boomy bass and flat highs - here are my thoughts on that, we did discuss this about 2 pages back in this thread, but for those just tuning in:
I've found the 641's bass performance is VERY sensitive to the placement of the sub. Also, even in the best position I could find for the sub I still had to "dial back" the bass control 3 little clicks from the default setting. Now the bass sounds clean, tight, and detailed. But it can boom if you want it to.
For my tastes, the highs are just right. The frequency extension is there, but the 641's are definitely not as "bright" as the ProMedias (which just screech to my ears) nor as "spitty" as the 560s. And the 641's midrange is much smoother than either of the other 2. Again, placement of the satellites makes a big difference.
The only complaint I have with the 641's is that the satellites are on the large side of things for the desktop and the bass unit is bigger than my computer case.
I did some measurements using a pink noise generator and sound level meter, taking measurements from the chair where I sit in front of my PC. Bass response was relatively flat down to about 35Hz (specs say 27Hz - there was output there but it was down about 8db from reference) and highs were relatively flat out to about 19kHz - that is excellent performance from a $160 set of speakers. The frosting on the cake was a measured clean peak output of 115db (again, measured in my chair which is "near field") listening to some old AC/DC. That's as loud as the loudest rock concert and can cause serious hearing damage in a very short period of time.
As everyone knows (or should know), sound quality is a personal thing. Everyone likes something different - that's why there are several hundred models of speakers on the market. So please don't take my word (or anyone else's word) on which speakers sound the best - you have to trust your own ears and tastes. Try and listen to as many different setups as you can - and remember that with ANY speaker set, the placement of the speakers in your room can play as big of a role in their sound quality as anything else.