Tad 1000's quiet?

There are several things you don't mention in your original post. If this a recent occurrence or has this combo always acted this way?

If it just started happening, you need to check for tubes going bad or gain knobs that got bumped or inadvertently adjusted downward.

I read up on the 150 Sig a bit, and the circuit can accommodate various tubes, specifically 12AU7, 12AT7, or 12AX7. These are all in the same family but have different amounts of gain; the 12AU7 has the lowest gain of the three and 12AX7 has the highest.

Is your turntable plugged into the phono inputs? Have you recently changed cartridges to a low output moving coil cartridge? I suppose that might not be the problem since you get the same low-gain problem with your iPhone, but it could be a coincidence. Check where you have the volume set on your iPhone as well as on the monoblocks.

The TAD 150 Signature's phono stage is made for high output cartridges. The OP's Lyra Clavis puts out a low 0.25mV.

I also wonder how you couldn't notice the totally tipped up treble and rolled off bass (i.e., completely unlistenable tonal balance) when connecting your turntable directly to the TAD monoblocks without benefit of RIAA equalization.

Based on your last post, you need to talk to the previous owner, your uncle. It's hard enough for us to ferret out the specs and characteristics of the discontinued standard model. Since it's a modified one-off, we have no idea what it does or doesn't do.