It's a great question.

Traditionally, water for coffee was defined in terms of hardness -- calcium, magnesium, and bicarbonates -- with an absolute minimum of anything else. The ideal was neutral hardness, about 90 mg/L equivalent of calcium and 50 mg/L of carbonates.

Nowadays it is far easier to test for TDS than for hardness, so there is a good technical reason to switch over. You can check water TDS with a ten dollar meter in a few seconds. Titrating, on the other hand is hard, time consuming and expensive; while test strips are not very accurate. In old neutral hardness standard translates to somewhere between 100 and 150 TDS, depending on the exact water composition.

Water softeners replace each calcium ion with two sodium. This leaves TDS readings unchanged, but makes the water non-scaling. In terms of taste, softened hard water is no worse than the unsoftened hard water, and it is better than overly purified RO water. If you install an RO unit and bypass some of the water, you need to get the hardness and TDS down to below 50 to prevent scaling. This will also taste acceptable.

Cirqua sells a system that uses RO water, bypass, and the addition of non-scaling chemicals, to get water up to 150 TDS without scaling problems. This will taste better than then the two previous softening alternatives.

They claim their proprietary non-scaling mix will taste as good as natural, neutral water; which is your question -- are all 150 TDS waters, or at least those designed by experts, born equal?

The claim has not been tested independently. I have checked out their home version. It's close enough so that it would take a lot more testing than I'm willing to do to really tell if it's the same or slightly different (their recipe cups a little different from Chicago water with the same brew parameters, but converges if you tweak the parameters -- this makes testing horribly hard). Since the difference between perfect water and conventional treatments is more obvious, their claims are close enough to truth to make their approach preferable.

I would like to point out that the taste differences between their treatment and the traditional competent water treatments (ion exchange, or RO with bypass) is smaller than their literature makes out. This make me hesitant to endorse their product more resoundingly.