Need a preamp?

As I already suggested to the OP.

Just use the built in attenuator as the volume control, and use an input switch box, like this
http://www.goldpt.com/sw4.html

If he doesn’t want to use them the next best is to turn them up full.

Use the $49 Schiit Sys, which has input switching, is 10kohm and a great match for his 270kohm input ST70 with 1v input sensitivity for full output.

And especially better again if the internal volume controls of the ST70 are completely bypassed.

The ultimate is go direct, and to use the source volume control if used at or above 75%, if not use the Schiit Sys as a preset so the source volume can be used at >75% with it’s remote control.

Cheers George

Many think that no preamp has its own colorations

Then they are kidding themselves

No preamp is "true to the source" with no colourations.
As the output stage of the source is just as matched to a poweramp input, as it is to a preamp input, in nearly all cases.

The "only" time a direct source to poweramp can get "coloured", is when the digital domain volume control is used down too low (<75%), then "bit stripping" can occur, which is a lowering of the 16bit resolution to 14bit or 12bit 10bit ect.

If this is happens then it's best to leave the digital volume control up full, and use a passive preamp, the next least coloured way of controlling the volume, and you can also "use it as a preset" to enable the digital volume control in the source to be used up much higher (>75%) then so it does'nt "Bit Strip" 

Cheers George   

But he needs a couple of inputs, and his amp is single ended.
But nice to know the  Nobsound Mini use a proper quad Alps 27 for the xlr instead of wiring pseudo xlr.
 https://ae01.alicdn.com/kf/HTB1FTx5LpXXXXbwXpXXq6xXFXXXj/Japan-ALPS-27-type-10KA-4-quadruple-Audio-p...

https://www.amazon.com/Nobsound-Fully-Balanced-Single-Ended-Pre-Amplifier-Controller/dp/B01MXC9HHW

Cheers George

except remember the volume control in the Schiit Sys is marginal

For $49 a screaming bargain, the Shiit Sys has an Alps 10kohm logarithmic mini, not as good as the Alps Blue Velvet or Black Beauty, but better than many around.
Same as this without the PCB cradle
https://images-na.ssl-images-amazon.com/images/I/51sGHNtCW2L.jpg
Cheers George

Would the Schiit degrade or color the sound more

Not if you get the internal passive in your ST70 completely bypassed (not just bridged out).
Then you’ll only be going through one passive, the Schiit Sys.

Cheers George

jaferd

Is there a reason I need a preamp other than the convenience of switching between the two?

No your Bob Latino ST70 has an input impedance of 280kohm and sensitivity for full wattage output of just 1v in.
With these sort of specs you only need a passive preamp

Would a ss preamp or passive preamp add to the sound? I presume any preamp would only degrade the quality and can’t make it better than it is.

Yes any active preamp will add colouration to the sound.

Just use the built in attenuator as the volume control, and use an input switch box, like this
http://www.goldpt.com/sw4.html

Or leave your attenuator full up and just get a $49 Schiit Sys which has two switchable inputs, you’ll be fine with your cd’s, and you’ll have enough volume with your phono preamp as well if it’s high enough gain.
https://www.schiit.com/products/sys

Cheers George