Hi Trevor, i recently had an Avant with a W76ERF042X44 tube, which is a RF32 VTR Avant.
The tube in your Grundig should be too big (W78 instead of W76).
Overhere in NL i know a guy who has replacement tubes nos avaiable for 100,- but although technically fine the pic quality is not as good as the original.
On itself with what you describe i don't think there's anything wrong with the chassis, but probably with the tube (only, bad enough already).
So far with 'glowing and tapping' i managed to fix already about 5 tv's, i don't know if you're ok with working on tv's and tubes since this is a very cheap fix (don't know how long it'll last but spending 100's of dollars is no option anyway imho).
I think i posted something about this somewhere here (was away for a month) but otherwise if you want to know you can pm me.
As for the width: did you chose the correct way ?, chosing 'format' and than 1 or 2 or 3 (i think 3 = 16:9) should fill the screen, and in 1 or 2 you can 'cycle trough' with the arrows on the beo4 between 4:3, 14:9 and 15:9 so not full screen (in other words: if this is your case this is correct, when looking at a 4:3 source signal).
When a true 16:9 signal is fed to the tv is should automatically change to full 16:9 screen.