bring a portable amp and your custom moulded IEMs and hook yourself directly to the soundboard
about those musicians earplugs... I bought a pair of them for about 40$, and while they mostly preserve relative frequency, they do roll off at high frequencies - something not so good for a grado/highs fanboy like me. maybe there are better solutions, I just have to find them...
oh, and about the attenuation: 20dB is often to much, except maybe if you plan to go to the Wacken openair festival. In my oppinion, concerts should be loud - louder than your average headphone listening - just not as loud as to damage your hearing. so 10-15dB at any sane concert should be enough to protect your ears. if a concert is quieter than your once-in-a-while rockout session with headphones, then it's not really enjoyable anymore. fortunately, my earplugs came with interchangeable filters, with about 10,15 and 20dB attenuation