Your preamp's tape input will be controlled by the preamp's own volume control, alas. One possibility is to get the phono preamp's volume control bypassed, and gain some sound quality that way. Two volume controls on the same signal would not be ideal, to put it mildly.
The benefit of using your preamp's tape input is that you go through only the "monitor" switch, not both through it and the preamp's "selector" switch. (I assume all preamps are set up in that way.) Not worth the price of two volume controls on the same signal, though, IMHO.
Does your phono preamp have enough gain to go straight into your power amp? Have you tried it that way? I'd rather change cable connections when I played LP's, and leave the preamp out of the system for LP's, but not convenient, I'll admit. Still, if the sound is a lot better that way ....