Tad 1000's quiet?

Yes as zd points out run the 3.5mm headphone/rca cable to the inputs on your monoblocks. If I am not mistaken the tad 1000 each have a volume/gain control. Turn down the volume first and then slowly turn it up once you have music playing.

Also want to make sure the power tubes are properly biased.

Ok. I'll pick up the rca to headphone cord and let you know what I find.

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.

Louder for sure but I can still turn the amp all the way up with iPhone turned all the way up.