I have just tried them out side by side, the 3800s were sat on a pair of Beovox 5000s so they were effectively on a stand, and the mid to upper range elements were, to all effects, at the same height as the Pentas.
First thing is that I was surprised at how lively the amp is, its a truism that a good amp can make average speakers sound better. I would have to agree that the Beolab 5000 amp is better than the Beomaster 6500 which came as news to me.
The Pentas are not as sensitive as the 3800s, which means you have to keep adjusting the volume as you switch between them. The Beolab 5000 is ideal for this as you can switch each set of speakers in and out independantly of the other, you can't do that on the Beomaster 6500.
It's as I expected, the 3800s have quite a bit more top end, and it can sound harsh when you turn them up, but they are very very clear. I was playing Alan Parsons Project - Pyramids and the harpsichord effect really comes out. Some instruments like cello solos sound uncomfortably harsh. I was expecting a bigger bass sound, given the size of the drivers. Its possible the crossovers need some work.They do sound bigger than you'd expect, but not dramatically so.
When you turn to the Pentas, the sound is much smoother, and you can wind them on a long way above the 3800s. The Pentas really have a punch when turned up, I was using vinyl, but I expect these would be very well balanced to suit remastered CDs.They seem to work better at moderate to higher sound levels.
The Pentas sound easier for listening, but for lower volumes go for the 3800s, as you wind on, you want those Pentas.
My opinion is that the 3800s are better for voice, acoustic and light orchestral where they bring out the detail, they are a bit cruel on vocalists who are not up to the mark, I don't think I'll be listening to too much Bob Dillon on these. You almost feel like the vocalist is singing to you personally so you could expect Tim Hardin and Dusty Springfield to show up well along with folk and blues folk, I bet western finger pick style music would be ok. These would match amps such as the Beomaster 3000 range.
The Pentas are better suited to amplified music, rock, electronic such as Chemical Brothers Prodigy, and big orchestral, huge film scores- imagine Bond themes, stuff with big atmosphere, maybe live music. As such you would think these would work best with more recent stuff.
The differance in sound between the Beomaster 6500 and Beolab 5000 is pronounced, I'm now wondering if the former would benefit from some work to replace the electrolytic caps or if its just a case of 'they all sound like that'.