Increasing gain for your vinyl front end when pre-amp doesn't quite have enough juice

What cartridge are you using? What is it's output voltage?
From your post, I am not quite certain what you are using now, except it's a Schiit Mani.  If at least I got that right, is the Mani a phono-only unit or does it include a linestage, which is to say, can you plug your CDP or TV into it as well, which would indicate it has a line level input?  If there is a built-in linestage, when you say the max gain is 59db (an odd number for a manufacturer to state), I assume you refer to total gain, phono stage plus linestage.  Yes? 

Once I get the facts straight, I can try to help.  We also need to know what amplifier and speakers you are using, and if possible the input sensitivity of the amplifier. (Defined as the magnitude of the input signal voltage required to drive the amplifier to its full output.)  But for now, if you are asking about the wisdom of driving another separate linestage with the output from the linestage already built into the Schiit, the answer is "not a good idea".

By the way, what is so terrible about a 70db S/N ratio?  For vinyl, that shouldn't be too obtrusive.

yes, my speakers are very inefficient Vandersteen Model 2C's from the late 80s. They need loads of juice to really make them rock. I just updated my Pre-amp to a Schiit Saga this past week, which is actually pretty awesome preamp (waaay better soundstage than the Onkyo receiver I was using as a pre before.) But the Onkyo had loads of gain, saga does not as it turns out.
My digital front end has no issues since my DAC has a built-in headphone amp (Audio-GD R2R-11) and can crank the knob and also set to high gain when needed. It's just the analog front end is a little light in the pants now.

By the way, what is so terrible about a 70db S/N ratio? For vinyl, that shouldn't be too obtrusive.

This^

Mulv:  "Doesn't your Mani have a 48 dB setting?" I do believe you are correct, I may have missed that setting and gone straight from 42 dB up to the highest gain 59dB. The dip switch gain settings on the Mani are a bit of a pain in the ass to deal with. (but god bless it, it's a good phono). I will play with that tonight when I get home. 

Lewm: to answer the questions: Nagaoka  4.5mV output MP-150. Mani only has a turntable RCA input and a single output. Amp is a behringer Pro EP 2500, 450 watts/channel into 8 ohms. I have the gain all the way up on these all the time. Here are some of the Behringer specs:
Voltage gain: 34Db
Input sensitivity: V RMS (@8W)  1.23V (+4.0 dbu)
Input impedence: 10kohm unbalanced / 20Kohm balanced (I'm using unbalanced

Didn't seem like there could be anything wrong with driving it at max gain from the phono stage, just not totally sure. That's why I wanted some tips/cautions/gold nuggets if possible from you fine folks. I love this site!

What preamp are you using?