Hi Mike,
Just a few weeks ago I made a shift from Penta III's to BL9's as front. I have BV7-40 with BL7.4 as you have and BL3's as rear. I actaully posted a detailed review of my thoughts about BL9 on the old forum, but this is of no use now.
I brief I can say:
Design: I am becomming to like the BL9 design more and more each day and they get the room to feel empty when replacing the much higher Pentas as I was expecting. Actally the fill the room designwise very good.
Sound - Low end: Not that much differet from the Pentas. After all the Pentas are very very good speakers. You will notis that they are more precise and goes marginally deeper. I don't have a BL2, but with the BL9's I don't thing your need it. Adding a BL2 could maybe destroy the very precise bass from the BL9's. The Pentas was a bit more old-school (not that precise and more warm)
Sound - Mid range: Improvement over the Pentas. Very clear and detailed without out beeing dominat. You will notis that you hear much more details in the music.
Sound - High end: This is where the big difference is. Again the BL9's are much more clear and detailed that the Pentas. The Pentas where soft and releaxed the BL9's are chrisp clear and detailed. And thanks to ALT this goes for the whole room with the same clearity and details. They are also perfectly much with the sound from the BL7.4 and you don't notis any change in sound when a speak / music move from teh center to the front speakers. The last part was my main reason for changing the Pentas with BL9's.
Overall: I'm very very satisfied but also impressed that my 10 years old Pentas actaully could compete with brand new allmost top of the line speakers. This shows how good the Pentas really are. In my mind the big thing is thet the Pentas was made when sound prority was different (more soft and releaxed) and the BL9's are made to fulfill the requrements of today (clear, precise and neutral). I will now try to find a room in the house where I can use the Pentas as I will not let them go.
I hope you can use this. I would also suggest that you switch the rear BL8000 to some with ALT if you listen to music in Speaker 4 mode. Otherwise you will notis a big big difference in the sound. The Pentas and BL8000 have the same warm releaxed sound, but the BL9's are very much different.
Ken