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The only option that may not be audibly transparent here is the tube stage on the Freya+. The SS Freya+ stage and the other two will very likely be impossible to tell apart by sound. Even the tube stage on Freya+ is more transparent than many tube pieces.
I would recommend buying what has the features you want. I love my Freya S. If you want tubes and a more robust product, i.e., no screen that can fail, get the Freya+. If you want a smaller footprint and outstanding performance, get the Pre90. If you like something about the Gustard, it will probably work well too.
Topping and Gustard are both excellent. I like the Topping brand better, but I woul recommend wichever has the looks, inputs, outputs, or brand you like. The objective performance is not really a differientiator between these two products.
I sent back a pre90 I bought. Super transparent, details in spade, but not my cup of tea as to tonal balance, flesh and 'relief' (See my post on the Pre90 thread). Just a matter of taste.
Using a topping Pre90 between a denafrips terminator and Den Apollo Amplifier replacing a Kinki Studio Pre and the extra detail is amazing. Well worth the investment.