2 channel tube preamp for a HT receiver?

I would like to improve the sound of a Harman Kardon AVR7000 receiver, connecting the CD and turntable to a 2-channel tube pre-amp and then to the receiver, for using the receiver just as an amp for stereo music, and keep the DVD connected to the receiver and 5.1 channels for DVD and Homecinema.
I have Dynaudio speakers (4 ohm - 87 db) so I need lots of amp current to drive them.
The receiver has 5 amp outputs and 5 preamp inputs (2 front, 2 rear and 1 center), interconnected with an external switch, as if the two sections of the receiver, amp - preamp, were internally separated and externally connected with that switch.
I thought to connect the 2-ch outputs of the preamp to the front channel preamp inputs of the receiver, but I doubt if, this way, I shall loose the front channels for DVD playing ( the DVD is connected to the receiver).
I also thought to connect the preamp to a L-R line input of the receiver, video 2 for example, and keep the receiver amp-preamp sections, but I think that this this way I will not get much improvement.
Do you think I can get noticeable sound improvement doing this way, or I am wasting my money?
How can I connect the preamp and the receiver?
Thanks