Bi-amping does make a big difference and even more so if you can get the 'next up' power amp in the range.
As i said before i have just added the Arcam Alpha 9P power amp back in to my set-up and the difference it has made is superb.
You cant go wrong with adding a good power amp to an already good integrated amp, especially to feed the different channels. As said above, use the integrated for the HF's and the power amp for the LF's.
With regards to going for a more expensive integrated, again it has its pro's and cons'
The integrated amps mentioned before' before someone threw their dummy 'outta the pram' are very good BUT they still reap benefits from having the power amp added, or if you can afford, monoblocks.
My girlfriend has the Cyrus 8v2 amp and 8 CDP along with B&W CM7 speakers. The system sounds superb and suits her taste in music down to a tee, thats why it took us 3 weeks to decide on the system after listening to Rotel, Roksan, Rega and Arcam.
She has listened to my system many times and yesterday notice an immediate improvement once the power amp was added and right at this very minute she is at Sevenoaks in Preston demo'ing the Cyrus 8v2 with the 8 power amp and the X-Power, i think.
We demo'd this a while back and she felt that it would be over kill, but seeing as she hardly watches TV anymore and just listens to the system, she can justify upgrading in the next 'more common sense' upgrade.