Hraouf1
The D80 is more detailed than the 3.8 but at the same time has a more refined midrange & top end. The bass of the D80 is another league complety which is deeper, has more definition, and authority. The D80 of course will play louder than the 3.8 and serves music like large-scale Classical much better. The 3.8 will tend to compress and sound stained in comparison. I owned the 3.8s for about 5 years which replaced my earlier designed Proac 3.5. The 3.8 was a huge improvement over the 3.5 in my opinion.
I have not heard the D38 so I cannot personally comment on it but the USA importer of ProAc in Maryland told me that I would be happy with the improvement of the D38 over the 3.8 but if I could afford the D80 that it would be the way to go as it is all that much better and worth the price. My dealer told me the same thing but was happy to sell me either one. I cannot say whether the D80 is worth almost twice the retail price of the D38. My room is large enough for the D80 so I went for it rather the the D38.
With regards to other speakers, I made an appointment with the local Wilson dealer and auditioned the Wilson Sophias in my own private listening session with my own music for several hours in one of their listening rooms. The amp was a tubed Conrad Johnson at my request (I also listened to them with Spectral gear). I was not impressed to say the least with the Sophias despite all the postive reviews. It may have not been the speaker's fault but something else in their system.
I also listened to the Sonus Faber Cremonas in two different HiEnd stores on the same day and was so impressed that I almost went for them. However ProAcs have always sounded great in my listening room and I had tubed Audio Research VT100 and a Reference 2 preamp at the time which of course are a good match for ProAc.
With regards to volume levels, the D80s sound great a low and high volumes alike. It doesn't take a great deal of volume to make them sound good and come to life. The D80 is supposed to be able to be driven with amplifiers as low as 10 watts but I have never tried anything less than 100 watts with them. Last year I replaced my 100 watt ARC VT100 MKII with an H20 Class D amp designed by Henry Ho of H20 Audio (www.iceh20audio.com) with a great improvement in sound over the ARC VT100. The H20 is the first non-tubed amplifier that I have satisfied with and gave me the illusion of tubes. It is rated at 250 watts. I've also home auditioned both Krell and Mark Levinson amplifiers with my ProAc Resonse 3.8s but they sounded too sterile for my tastes.
Of couse the sound of any speaker will depend on the associated gear, proper matching, set up, and of course the room too.
Hope this helps.