Tracking error distortion audibility


I recently unpacked my turntable from a couple of years of storage. It still sounds very good. Several times during playback of the first few albums I literally jumped from my chair to see which track was playing as it sounded so great. After a while I realized the "great" sound was always at one of the "null" points. They seem to occur at the approximately the proper place (about 125mm from spindle) and near the lead out groove. Questions:
Is this common? I have improved the resolution of my system since the table's been in storage but I don't remember hearing this before.
All others geometric sources of alignment error not defined by the null points (VTA, azimuth etc.) are essentially constant through out the arc correct? If so they should cancel out. I assume the remedy is a linear tracking arm but I am surprised at how obviously better the sound is at these two points.
Table - AR ES-1, Arm - Sumiko MMT, Cart. - Benz Glider, Pre - Audible Illusions, Speakers - Innersound electrostatic hybrid
Do linear arms really sound as good across the whole record as I hear at only the nulls with my set-up?
feathed
Using a good protractor is essential for proper setup. John Elison has created a superb spreadsheet to aid setup, available online in a couple of places such as Enjoy the Music (bottom of page).

Among the tools I use is a Feickert protractor for setting overhang and horizontal alignment, as a starting point. This is followed by the Feickert computer software to optimize the alignment for best performance in situ. People have used oscilloscopes for similar analysis; the Feickert program was expressly written for the purpose and gives clear visual indication of performance, enabling fine tuning of parameters such as azimuth and damping. Due to manufacturing tolerances the stylus may not be perfectly aligned in the cantilever, but this is no longer an obstacle since the software removes the guesswork.

Dealer disclaimer.
I have never used an arc protractor, but to me it just makes more sense. Put the tractor on the platter, sight it to the pivot point, immobilize the platter and tractor, align cart........ None of this align cart to point A, rotate platter and align to point B, repeat. Having a fixed and stationary datum, will always result in a more accurate alignment.

"Feathed, how are you verifying pivot to spindle distance?"
Dan_ed
Unless one needs the distance for making an alignment device, the pivot to spindle distance is of no relevance at all.
Dear Feathed: I can't understand your last post and maybe I'm wrong but making a quick revision on my original tonearm jigs to mount the tonearm in a TT in all cases that distance determine the exactly position of the tonearm in the TT. Am I missing something here?, please explain about.

Regards and enjoy the music.
Raul.
For this discussion ignore the arc protractors. Universal protractors work for various spindle to mounting hole distances correct. If you have slotted headshell mounting holes you can adjust cartridge angle and overhang correct. If you drill your mounting board hole further from the spindle than manufacturers specs, simpling slide the cartridge closer to the end of the tonearm and adjust angle accordingly. You get the exact equivalent of a longer effective length tonearm. You obviously need to allow for slight differences in mounting hole to stylus geo in case you change cartridges but they vary only slightly. Brooks Burdan (sp) explained this to me 20 some years ago. It's so simple I just assumed everyone did this. I've never given it a second thought.