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

Honashagen,

Your question is essentially that you are asking what level of quality you need to attain to improve on Oppo 105 direct to your DNA amp. (Or possibly how to find it, but the answer to that is obvious, trial and error, or, asking an expert with more experience, hence, presumably your post.)

The discussion up to now seems to me to be ignoring one obvious aspect. That is, what you are really trying to do is improve on the built in preamplifier that comes with your OPPO 105.   To really and truly bypass your Oppo 105, you’d pretty much have to take a digital out signal and go with a preamp that takes the digital signal and modulates that into the analog domain. Otherwise you are using some of the preamp circuitry of the OPPO already. 

I believe (and I may be wrong) that your Oppo uses different pathways for its RCA outputs whether line level or variable out, than it does for its XLR outputs.

It means you have potentially two different quality preamps built into the Oppo, and you’re listening to just one of them.

Consider a preamp might outperform one, but not the other. 

For example if if you had a Bryston amplifier with both RCA and XLR, you might prefer one to the other, and require a preamp taking in the digital signal to improve the one you don’t like to get to the level of the other.

Before I’d blindly assume a preamp which is just as likely to color poorly as well as improve it, I’d also equally consider an amp with true balanced technology to see if the OPPO does a better job via its XLR outputs. (ie you may already own a better preamp, you are just not using it)

I guess I’m just pointing out you already have a preamp in the OPPO.  

Atma-sphere MP-3 might be another good alternative. :)

honashagen, The preferred hookup for the Pass Labs with it's super
symmetry circuitry is via XLR, since your amp doesn't accept balanced interconnects you won't be optimizing the capabilities of the Pass Labs pre. The McCormack is a fine amp and perhaps even the ideal match for your system, but it seems to be the bottleneck for what your trying to achieve. Perhaps the money spent on the Pass pre might have better been spent on an amp that plays nice with the Oppo?

Agree with fsonicsmith re jump factor. 

Answering the OP question, I'd say $5k to $20k. In other words, the price of a Shindo preamp. 

I would rather go straight from DAC to amp using jriver digital volume control than use a $2k preamp. The big jump, where the preamp adds musicality (and not just warmth) happens above $5k. 

Warmth is often just smearing and roll off. Musicality is extra tone, no details smeared away, and extra compelling transients. 

Buy the New Schiit preamp at $699 using 6sn7  tubes, 
justreplace them with the Golden Lions ,and you have the =
to preamps up to 3 timesthe cost, even a solid state option if you choose to  but a Pangea sig, MK-2 power cord. Fantastic for under$900 maxed out .also upgrade the fuse if you choose to.