KEF themselves have been using Marantz at shows and exhibitions, so they obviously feel there's a synergy there that is hard to ignore. From all the amplifiers we've tried (Marantz/Yamaha/NAD/Rotel etc), we tend to agree. In addition to that, we've not really found any amplifier to not work well with the Q300's, so your Brio 3 will work fine with them as they're very easy to drive. I would go with the Rega or the Marantz.
Your NAD 3020 will sound fine with them, but they will appreciate something with a little more oomph.