Power first then pre. If you look at the two sections of your integrated as separate components, the pre section (which is almost as good as a decent separate pre) when combined with a really good stand-alone amp will come pretty close sonically to a complete 'separates' system.
Whereas a new separate pre driving the amp section of the integrated won't really get to shine, and the result won't sound much better (if at all) than the integrated does now IMO.
And besides, if you can possibly try a few amps using the pre in the integrated, you'll likely be able to make a good amp choice. On the other hand, were you to try a number of different preamps driving the integrated's amp, the pros and cons might not be very obvious that way. Better to wait and audition preamps after you've made your amp selection. That would be my advice.
Whereas a new separate pre driving the amp section of the integrated won't really get to shine, and the result won't sound much better (if at all) than the integrated does now IMO.
And besides, if you can possibly try a few amps using the pre in the integrated, you'll likely be able to make a good amp choice. On the other hand, were you to try a number of different preamps driving the integrated's amp, the pros and cons might not be very obvious that way. Better to wait and audition preamps after you've made your amp selection. That would be my advice.