How much do I need to spend to get a preamp that sounds better than no preamp?

When I inserted a Hattor XLR passive (sans buffer) between my Oppo 105 and my then EP 100.2SE amps(newest AA amp on its way) everything improved, especially bass

It could be that my system is 100% balanced, Isn’t everybody doing that by now?

It was far more likely that you were experiencing "bit stripping" (a loss of resolution) when you went direct, as you were not using your Oppo’s volume control above 75% as the level was too loud to do this.
And getting the better bass with the Hattor Passive pre you had you Oppos volume control up higher so there was no "bit stripping" going on.

And as far as balanced sounding better than single ended this is a Furphy, the only time this may happen is if the interconnects are very long (over 5mts> 10mts)

Cheers George

And as far as balanced sounding better than single ended this is a Furphy, the only time this may happen is if the interconnects are very long (over 5mts> 10mts)

This statement is false- balanced lines offer less coloration even if they are only 6" long. This is way you can run longer lengths with them, since they have far less coloration. Just today I installed balanced cables in a system that were only 1/2 a meter and the improvement was instantly audible.

balanced lines offer less coloration even if they are only 6" long

6" This is false, your either hearing things or your biased toward your own retail preamp product.

In many cd players or dacs I've seen the balanced output is just an added opamp changing the "real' single ended circuit to balanced. Same goes with the input of many poweramps, which btw are single ended output to the  speakers. 
The only time balanced is better from source to to pre, or pre to power is for noise cancellation with interconnect over 5mts>

Cheers George  

tweak1
It could be that my system is 100% balanced, Isn’t everybody doing that by now?

Here is Nelson Pass’s take on balanced vs single ended.
When asked by an owner what sounds better on his X350.5
On diyaudio.com

Nelson Pass The one and only Join Date: Mar 2001
"Except for noise, it gives pretty much the same performance, and this is
because the inputs are summing junctions, and single-ended inputs are merely another balanced source with ground as the (-) input."

Cheers George