Its not a bad idea, a lot of people do that, but there are pitfalls.
Some CDPs use a digital volume control. Essentially, they subtract bits from the total so as to reduce volume. So as the volume is decreased, the signal quality suffers from reduced resolution.
Add to this the fact that the CDP puts out too much voltage (as required for some insane reason by the Redbook spec), such that it can clip any amplifier made and this means you have to run it **all the time** with the volume less than full!!
So one method used is a passive volume control interjected between the CDP and amp. The problem here is that the series resistance caused by the control can interact directly with the low frequencies and the extreme high frequencies of the CDP (the latter having a lot to do with the interconnect cables used) which can result in colorations, including loss of bass impact and overall dynamic quality.
However in practice many people are alright with this, likely because they've not heard what a good active preamp can do (as opposed to a poor one) in their system.
To get away from the problem of cable colorations, an active preamp is used, which restores the bass impact and reduces colorations, despite the fact that an active circuit is used. How successful this works varies quite a bit as not all active preamps are created equal and some don't work as well as none at all. Hence the active/passive debate raging on this and other audio websites.
So you do have to be careful (if you want optimal results) regardless.
Now if your CDP has an analog control (some of the Oppos do) and its good at controlling the cable, then this strategy might work quite well.
I advise doing a bit of audition if getting the most out of your system is the goal. If you just don't care then just hook up the CDP directly and call it good :)
