hmmmm this is getting into an area I don't understand...I am not sure if cds do overload an amp,
cds put out 'nominally' 2 volts, which is the industry standard, now some cd players put out a lot more, some less, so if you do a side by side comparison, you will think the louder one sounds way better.
If you don't understand what is going on, the wool will be pulled over your eyes, and you will gravitate to the loud one, and the quiet will sound awful, thin repressed.
Some speaker makers know about this and do it to their speakers, kef for one, and in the shop kefs do sound impressive for that reason.
The 2volt output of a cd then goes into the pre, or the integrated, where it goes via a swithing network : a relay on posh amps, a selector on others, into either a buffer stage or through the volume control.
Now the vol will 'reduce' that 2 volts to a fraction of it, so you might have a signal of 1/2 volt or a lot less.
The input sensitivity, say 300mv, means the amp needs 300 mvolts, or 0.3 volts to drive it to full power, so the gain of the amp is the voltage out /voltage in, we know the volts in, 300mv for full volts out, if we measure full volts out, we know the gain.
volts gain = vout/vin,
vin = sensitivity
vout, you can get the power supply from...
exceeding the psu rails causes voltage clipping
so anything less than 0.3 volts will not produce the full power of the amp, which is good, as it would be deafening.
This is where I get a bit confused, the headroom is to do with how many volts you can put in, but you see that an 'attenuated' cd will not drive it to full power, so not causing overload or clipping.
Some power amps are insensitive, requiring 1 volt in to full output, so low output sources such as tape/tuners may not drive them to full output. So we need a pre-amp to give a gain of around 3.3 ie 300mvolts x 3.3(gain of the pre)=1 volt, to enable the power amp to be fully driven.
This is the rationale behind some active pres, tho passive can be used if you have cd( just a volume control to attenuate the signal) it always sounds better to my ears to have a bit of boost.
That way you'll lose less sleep worrying over which, the pre or the power, has the most affect on the final sound. Additional advantage also are that you'll free up some rack space, be less troubled by interconnect and power cable choices, being troubled by other choices such as whether to use a passive or active pre, oh and you'll save some money too. 
boo hoo ,don't say that Henry,Ive already got a valve integrated,just really liked the Conrad Johnson when I heard them,it has an attached power cord already,so no worries there,and have heard nothing but praise for the bel cantos,just was curious as to which was influencing the sound the most.
