This is EEVblog. Buy a 500MHz scope, hack it to 1GHz, build a rise time tester and post screenshots

55MHz is way above audio signal bandwidth, but you will encounter such bandwidths in opamps used for audio these days like LM4562, OPA16xx or the good old '637 because Miller compensation is brutal on them - 55MHz is what it takes to get 55k gain at 1kHz and one needs that gain for specs, particularly if driving external buffers.
A 300MHz scope seems like an overkill, though.
Transient response testing of supplies above a few hundred kHz will really be testing of transient response of bypass capacitors. I doubt you need to test at hundreds of MHz unless you work with RF or digital circuitry which will actually draw such ripple currents.