Tad 1000's quiet?

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.

Combo has acted like this since I bought them both at the same time. I have a Lyra Clavis dc cart on my Basis Tt with a sound smith ruby retip. As I said it is louder bi passing the 150 but I would still guess turning them all the way would be an impossibility. Do all tube amp owners own a tube tester? I'm guessing it's the only way to know if any are bad? I can list the tube types later this evening if that would help.