Hi GD. I can totally empathise with your comments, when you fade from vinyl-cd you will notice a "coldness". There are several ways you can install a "tube-buffer" to your set-up.
One is, as you suggest, to use the send-return/loop. This has an advantage in that you can turn the tubes in/out of the signal-path as you require them. However, there is one disadvantage in that you will be putting the signal through more circuitry as you would be if you installed the tubes between cd/input. Even if you use them in the latter-method, you will still notice a "difference" in sound between phono+cd, though the cd will be "warmer" and more organic, it will still sound different to vinyl.
This leads to the other method (IMHO the preferable way, bearing in mind that 9/10 12"s are mastered from cd these days anyway, albeit through some tube-compression maybe). This would be to put a tube-buffer on the main-output of your mixer. This will put a "tube"-signature on everything, and therefore it will not be so noticeable when you fade from vinyl-cd. I am not sure what devices there are on the market for this job, I wish I did. You need a very simple tube-buffer. If you know a good tech (maybe you can assemble stuff yourself?) my suggestion would be to trawl the 'web for a BROADCAST VALVE LINE-AMP SCHEMATIC. Being designed for broadcast it will easily have the headroom to accomodate the levels from the master-out (typical broadcast gear runs at +18dB). A good tech should not charge you too much to assemble it, and it should not cost more than $200US in parts, providing you are shrewd where you go to buy them. Try a search in Google for "TUBE LINE-AMP SCHEMATIC". If you don't get any joy post back here and I will see if there is anything I have that I can send you.