If you buy just a power amp, your still stuck using the Marantz pre/pro as your preamp and even in so called "pure direct" mode, the preamp in HT equipment is just a mere afterthought.
So go for the integrated instead. The stereo preamp section in PM14S1 is far superior than the one in your Marantz pre/pro. With its true 4 ohm rating it will drive most speakers out there and with its "Power Amp Direct" feature, it allows for seamless integration into your current HT system. And since your going to add a TT, its phono stage is excellent too.
BTW, I think the Marantz TT-15S1 with its Clearaudio Virtuso Ebony cartridge is a fine choice. It would be hard to find a better combo at this price.