Double Bass Suggestions to Replace Spirocore Medium High C string on a 5/8th bass

Well, the high C is bright. The reason why I don‘t use it is that the tension is rather high on a 110 cm scale and the string is thin and needs a lot of pressure to fix it onto the fingerboard.

General rule: thin strings Sound good in higher positions, thick strings sound good in lower positions. So something in between might work well.

For a (initially) bright string there is the tone filter (from Pirastro but also Super Sensitive and maybe other manufacturers) which is basically a rubber disk with a hole in the center.
Put it between bridge crown and string so that the bridge groove is in the center of the hole and put the string under tension. The rubber will then press against the underside of the string and damp higher partials quicker.
If the string calms down, you can remove the rubber disc for a medium bright sound again.
 
You don‘t want that. It‘s an extremely bright Flat-Chromesteel and his fingerboard is specially shaped so that the string height is extremely low just for this one string.

There are not many options for nice to play high C strings. A Corelli 379TX is not too bad but might be a bit lower tension than the Spiros.

A lot of interesting high C strings have been discontinued now (or are soon).
Thomastik has a Spiro Weich high C string that is special order.

I stick to my Innovation prototypes that I like for high C, G and E. The Braided are not bad either, just a bit darker, but the old Aluminium wound high C doesn’t fit the sound and tension of the regular set and the chrome steel wound prototypes could have had a bit more tension. The Velvet Compas 180 high C was badly manufactured for the whole batch and it seems that Velvet silently vanishes from the market in the near future. The Obligato is rather thick and doesn’t sustain well pizzed in higher positions. Steel core strings are thin and cut into the finger with medium to high action.
Wound gut might work with Spiros like Oliv, maybe with a G too. But expensive and can break.
Maybe a Zyex high C. Never got one. When I was searching they didn’t had a high C, only the regular set.