What gain/impedance issue? Impedance looks fine, the output impedance of the SLP-05 is 400 ohms, while the input impedance of the 302e is 100Kohm SE and 200Kohm balanced. No issue at all there.
The Cary gain of 17 dB single ended and 24 dB balanced is a tad on the high side balanced, it still should work fine.
You can certainly find preamps with less gain, though they will have different sonic signature from the Cary as well.
I would think that you would be better suited by how the preamp/amp sound together then by worrying about how they look next to each other on a spec sheet. I understand impedance matching, to a point, that is no concern in your case. Gain can be adjusted by attenuators, like those available from Rothwell.
The Cary gain of 17 dB single ended and 24 dB balanced is a tad on the high side balanced, it still should work fine.
You can certainly find preamps with less gain, though they will have different sonic signature from the Cary as well.
I would think that you would be better suited by how the preamp/amp sound together then by worrying about how they look next to each other on a spec sheet. I understand impedance matching, to a point, that is no concern in your case. Gain can be adjusted by attenuators, like those available from Rothwell.