Proac Rake angle?

You have to experiment to find out what works best. I would start by moving your head up and down and see what happens.

I agree with sfall. I would keep them level and get a different chair if you determine the height makes a difference requiring remedy.

In my limited experience, although I've been impressed with some speakers that have utilized rather noticeable rake angles, I've found no improvement by changing that on my own speakers. But I have found most speakers sound best to me a bit higher up than is commonly recommended. (I'm just under 6')

As the M-T-M design of the D48R may limit the vertical flexibility of speaker/seat placement even more, you may find it worthwhile to try it out.  (The advantage being less overall room reflections, I believe.)

I would contact Pro Ac - they will let you know which practice is best

Tilting them can upset this alignment - depending on the speaker deign. They could end up sounding much worse in other areas

Do you hear a difference if you align your ears with the tweeter?
- Try standing and then sitting and noting the difference in image.

If you do hear a difference - I would suspect reflections from the ceiling or side walls are causing the change

To be able to provide better feedback...
- how high is the ceiling?
- How far are you from the speakers?
- How far are they apart?
- How far are the center of the drivers from the
>>> wall behind them?
>>> side walls?
- Is the floor between the speakers and yourself and the speakers carpeted?
- Do you have the ribbon or dome tweeter

I believe the dispersion patterns from the drivers should compensate for a couple of inches discrepancy on listening height. ProAc's have a pretty good reputation for being easy to place (within reason). 

In practice, I've never heard any difference whether seated or standing with the Pro Ac models I've listened too, but they did not have two bass drivers

I've tried tilting with my own Gershman Acoustics speakers, but I found they were better when virtical.

The other hing I've found impacts imaging is the furniture in the room - e.g. I had a 7ft book case in a corner behind the listening position. There were some serious reflections happening the top of the book case and the ceiling, which caused smearing - A piece of 1" foam on top of that book case cleared that up and improved the imaging.

Speaker placement and furniture are critical when trying to achieve the best image.

Regards - Steve
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Thanks everyone for your responses. Williewonka, I emailed proac and Stewart replied and advised not to tilt, and  to get a more ideal seat.  Will do.  On a side note, I will say that these have truly started to sound magical.  FINALLY.  I was worried about them to be quite honest, and thought I had made a mistake with them.  They sound so much better than they did upon first hooking hem up.   For the last 3 days, they've been playing Mahler, so that probably loosened them up right there. It is true, they need 500 hours before you can judge them.  They almost sounded defective at first!  What a difference.  These really do cast a spell on the listener. I'm simply amazed.  With 550 watts Mahler has never sounded so real.  The dynamics and just shear prowess of these speakers is astonishing.