I have to hand it to you for doing such complete analysis on your must. If every winemaker were as conscientious as you are, we’d have fewer stuck fermentations, sluggish malolactic fermentation bugs and unhappy yeast beasties. I think you made the right choice to not add any acid to this batch. As you mention, your pre-fermentation TA is already around 0.6-0.70 g/100 mL, which is definitely in the red winemaking sweet spot, especially for Napa Valley Cabs (St. Helena is a small town at the northern end of the Napa Valley). The pH that you report is a little odd though and gives me pause. With a pH that low (3.02) one would expect to see a corresponding TA in the realm of 0.80-1.0 g/100 mL as opposed to the more reasonable, lower levels you saw. My suspicion is that you already pointed out the source of this weird parameter – the fact that you only standardized your pH meter with one of the buffers, the pH 7.0 buffer. When using a pH meter (a great tool for serious winemakers, by the way) it is absolutely critical to standardize (some say “calibrate”) the pH meter before every time you use it and multiple times a day (say, once every four hours) if you are running a lot of samples. pH meters have super-sensitive membranes and measure pH accurately only if they are “shown how” to report their accurate values within a measuring range by exposing them to pH 4.0 and pH 7.0 buffer, in the case of making wine measurements. I suspect that your pH for the current batch was really in the realm of 3.50-3.60 and because you weren’t able to properly standardize the meter it gave you an erroneous reading.
For future picks, however, if you have any control over the matter, I would try to get your Brix a little higher. 2006 was definitely the kind of year where one could pick at slightly lower Brixes and still get good flavor development but 23.72 is pretty low for a full-bodied Cabernet Sauvignon. You just won’t get the alcoholic heft, ripe rich flavors and the mouthfeel that many people are looking for. Depending on flavor balance of course (waiting longer if flavors are still green but picking sooner if they’ve developed past that point) I generally try to pick my Napa Cabs around 25.0–26.0 Brix. This ensures that the acid levels have a chance to come down, tannins can ripen and the dreaded green bell pepper aroma (caused by a class of compounds commonly found in Cabs called methoxypyrazines) will disappear from the ripening grapes. You also should get some really nice rich blackcurrant and blackberry flavors and aromas which will yield to coffee, leather and tobacco as the wine ages as long as your wine is kept in oak barrels or you use some kind of non-coopered oak like beans or staves. Sometimes waiting longer to pick does mean that you need to add acid – just don’t add too much. Since judges have mentioned your wines lacked balance and finish, I would suspect that they might be too acidic (based on what I infer about your past acid addition practices) and not ripe enough. Properly ripened Cabernet grapes from the Napa Valley can indeed make some of the best wines in the world.
Again, I applaud you for buying fresh grapes and being willing to tackle the many challenges that grape winemaking can and will present. However, as you gain in experience you will also be able to reap the rewards that employing only premium winegrapes can bring.