Help Me Improve My Turntable Set-Up

The Saga+ has no gain. The Aegir has an input sensitivity of 1.2V, but the Mani output should be about 300mV for a standard MM cartridge. It’s not a big surprise that you aren’t getting much life out of the Aegir. Even with low efficiency speakers it should do okay unless you’re asking for high volume. The dual mono Aegirs might help since you’ll need to go balanced to do it, and most likely use a pre with gain.

i’ve run the Saga/Aegir into 87dB floorstanders and filled a big open room to respectable volume with aplomb. However my DAC outputs 2V.

I think you have to try to find some opportunity to listen to someone's system that sounds the way you want yours to sound and work from there.  It sounds like you're just generally unsatisfied and you're guessing at what it might be.  Unless you know what you are trying to achieve, it's impossible for anyone to make suggestions, no matter how well-meaning. I have the Harbeth P3s as well and I would describe the sound as very natural and realistic for their size. With the right recordings, they can sound like live music, but I think that's not what you mean when you say "come alive".  

Are you analog only? If there is a dac too do you feel the same re shortcomings from both formats?

I agree Harbeths are tough on amps ( which is not a great trait for a loudspeaker imho ) although they can sound nice.

Schiit gear is decent for the money but nothing more. Give your super expensive book shelves a better amplifier.

I have to agree with church man. Schiit is really good for the money, but they are really budget stuff. If you want to get what the Harbeth’s are capable of you are going to have to significantly upgrade all your electronic components.