While not positive, the adjustments you made may have thrown off the blue gun a little bit. The Red & Green guns firing at the bottom at full power are hitting fine, but the blue gun seems to not be hitting it correctly or has had an adjustment that is tweaking the level at the bottom. Red + Green = Yellow - so a lack of enough blue is the issue at that part of the screen. It does sound like burn in and if you have been watching 4x3 content for years, then an adjustment of your guns could have raised the red & green a little bit. So, they are firing with more power near the bottom of your screen because they are using the 'unused' part of the CRT gun. ALL CRT projectors experience burn in. With a 4x3 projector that does not experience a specific section with burn in, the entire tube will be burned in in the 4x3 shape.
As for LCD - I am not aware of a NEW form of LCD coming to the market. There is already LCD flat panel displays and rear projection displays. As far as I know, those are the only two that are out there and are going to be out there for a long time.
In my opinion a good LCD display from a good manufacturer produces a crisper, cleaner image than plasma does... at a premium price.