Lucky,

I am an audiologist (and very amateur muso).

For live music, particularly when you are playing, I would recommend custom made plugs, as they will be more comfortable, and you're more likely to insert it properly , to get the required attenuation (maintaining an acoustic seal will be much better on a custom plug as it will contact a greater surface area of your ear canal).
In my experience, people rarely seem to use off-the-shelf solutions correctly. How often do you see people with earplugs virtually hanging out of the ears, contrary to manufacturer's instructions?

Unless you are doing rock concert-loud gigs, I would go for ER-15 attenuators, as they will preserve more of the high frequencies needed to give the crispness/clarity you need. For raw attenutation, sure, 25 dB attenuators offer more protection, but then if they muffle too much while you're playing, you'll end up taking them them out -and get no protection. You can get different attenuators to put on the custom plug if you decide the attenuation level is not right for your application.
And just to clarify, no ear plug will offer "flat" attenutation, rather, 'flatter' attenuation than full ear plugs. There is still a little high frequency roll-off.

I'm not sure what is available to you in your part of the world, but one make of earplug that I have fitted which is very nice is made by 'Phonak.'

Personally, I have ER25s which are great for really loud gigs but for more moderate gigs which are a bit too loud they are overkill. I feel like I'm missing too much top end with them on. I would prefer ER 15s for my usage.

Hope that helps.

Eugene