REGA VTA Question

The arm tube was exactly parallel to the platter when using REGA's own Exact 2 cartridge. So, I am having difficulty understanding why REGA recommends doing nothing when the arm tube is not close to being parallel to the platter.

Maybe you've answered your own question. If a non-Rega cartridge doesn't perform as well, people who don't think things through like you're doing may just go back to a Rega one, without trying to do a better setup.

Rega also recommends playing LP's without cleaning them. Unless they get a commission for each LP that has to be replaced because it was ruined, that advice makes even less sense.

Roy Gandy recommends what he recommends, for whatever reasons. You can ponder his motivations or you can seek your own answers and learn to optimize your own setup, as Viridian suggested.

Gandy stresses mechanical ridgidity over all else. They feel that Installing a shim or variable VTA adjuster nut compromises the sound and Gandy argues raising the arm up or down a few mm's actually only changes VTA angle by an inconsequential amount. Sometimes it's no bad thing to have the arm a little lower towards the back, see how it sounds to you with no spacer first. From rega's site:
Arm Height or V.T.A adjustment can be a controversial subject. Rega believes that the integrity of the arm fixing onto the arm mounting board is much more important than the questionable facility of arm adjustment: The arm should be reasonably parallel to the record surface or slightly lower at the mounting. The only time a spacer is necessary to raise the arm height is if the rear of a cartridge is hitting the record whilst playing.

I use a single 2mm. The ideal would be 3mm. Does not exist. 4 mm raises the arm at the pivot point too high. Gandy himself says is preferable to be a bit low.
VTA is not really that important except if you have many 180 or 200 grs records badly warped..
The most important improvement you can make is to get a decent constant rotational speed. You must replace the polymer sub-platter by an Isokinetic (or Groovetracer, if you are in America). Even if you have the TTPSU the Rega is not speed accurate. And the plastic subplatter attracts dust to the belt. Complement that with the Rega white Belt
Buy a standard (not the Rega Version) Michell clamp. This comes with a felt washer. Keep it in for normal records, remove it for 180 grams or over. It's the equivalent of raising the arm pillar pillar about 1 mm. Get rid of the felt mat for a further rigid coupling of the record to the platter. I found Herbie's Mat which is silicone foam a good one.
And then forget VTA. Make sure records are immaculately clean, by using a carbon fibre cleaning arm and connect the wire to the ground of your preaamp. This carries the static out which prevents dust atraction, and this way your expensive Glider stylus will last maybe one year more.. Nowadays they are hard to find. Try Analogue Seduction on eBay.co.uk.

You said:
"So, I am having difficulty understanding why REGA recommends doing nothing when the arm tube is not close to being parallel to the platter."

Fillmoor answered you question with great accuracy, above. If you want to read it in Roy Gandy's own words, go here:

http://www.n.mackie.btinternet.co.uk/documents/regaonvta.html

You MUST buy a VTA adjuster for non-Rega cartridges!

I recently purchased a near mint Rega P3-24 for $400 (minus the owner's Clearaudio cartridge which was listed separately)to upgrade from my 90's model Thorens TD280 MkIV.

The Thorens (which came pre-mounted with an Ortofon MC-X5!)was purchased new for $295 from J&R Music because I was afraid turntables would disappear from the marketplace at that time. It sounded great for all these years with the Ortofon, despite the tonearm being canted down several degrees from horizontal (positive VTA).

After installing the MC-X5 on my "new" Rega, I was dismayed to find my music now sounded dull, muddy and lacking definition. At first I feared that I had damaged the cartridge during installation, or that it was not well suited to the RB301 tonearm. Upon closer inspection I determined that the tonearm was actually canted slightly up(ass down)from being parallel to the record surface.

Research revealed that Rega manufactured cartridges are only 14mm high from headshell to stylus tip while my Ortofon measures about 19mm.

I ordered the Rega 2-4-6-8mm VTA adjuster. Elsewhere in these forums I read that if you raise the RB301 by 4mm it creates a problem in that the counterweight hits the shallow top of the dustcover when closed. This can be solved (for over $100) by replacing the counterweight with an underslung aftermarket or Rega tungsten weight of smaller diameter. Thankfully this wasn't necessary!

I installed the Rega VTA adjuster to raise the tonearm base exactly 4mm and checked the counterweight clearance. Because the Ortofon MC-X5 is so light (4.1g), the stock Rega counterweight balances very close to the tonearm pivot and thus does not rise quite high enough to contact the dustcover when the stylus is cued to the record. While I lucked out in this regard, I caution other Rega owners to take this (cartridge height and mass) into consideration if selecting a non-Rega cartridge.

The result - I am blown away by the new sound of my vinyl collection after this small adjustment. The Ortofon MC-X5 now sings better than ever regardless of whether I'm listening to jazz, rock or opera.

VTA makes a HUGE difference despite what the Rega folks may claim.