Just how portable are we talking?
Behringer XR12 (circa 250 quid if you shop around) is what is coming to my mind at the moment for all your processing needs in a pretty small package. Will give you all the control over your EQ, fx and dynamic (e.g. brick wall limiter if you want one and all compression in-between) that you'll need... and a 31 band eq on your output to tune to you output device.
You could even sack off your head and use an AUX from the XR12 to a power amp and that can become your bass rig. Much better tone shaping than your typical standard bass head. Use the other AUX (pre) for DI to FOH. Anyway, I digress.
If you wanted an extra from the headphone output, slave something like a dedicated Fiio headphone amp - but your 64s should be happy on the Behringer.