I like XP-10 a lot (granted never had a tube pre in my rig and that's one of the things I'd like to try sooner than later). Haven't heard XP-20 but based on my experience with the 10 and what ppl told me, 20 should be an amazing piece. Obviously XP pre-amps offer truly balanced circuits (and other benefits of having a good pre - soundstage, noise level, etc) over the volume control of INT30A or INT150, but it's subjective if the added cost is justified. XP pre also has HT by-pass, one of the features I need. INT models do not. One thing to note here also is it sounds better when I have my AVR passed through XP-10 pre to XA30.5 using balanced connection than connecting AVR to XA30.5 unbalanced directly. My AVR doesn't have balanced outs.
If you don't need HT by-pass and want to get an integrated than the separates, I'd go with INT30A and use the rest of the fund towards upgrading from 1007Be to either 1028Be or Diablo Utopia. :)
For a tube pre to pair with either XA30.5 or X150.5, I'd make sure the pre's output impedance is <1k ohm across the board (not just the rated output impedance).