The HE-400 is complementary to the HD598. The HD598 is fairly neutral with a slight upper-mid emphasis to give it some pizzazz. The HE-400 is bassy and sparkly/airy with a recession in the upper mids. You may find trumpets, snares, etc. lacking a bit of bite and edge with the HE-400. There are cheap mods such as the Jerg pad mod which improve on the HE-400. I have tried both with the Vali. What really sets apart the HE-400 is its bass quality. A very tight and fast bass - possibly the best quality in the entire HFM lineup without having to resort to a power amp - and even then, it's still debatable that HE-5 or HE-6 bass quality is better. Both HE-400 and HD-598 work great with Vali. I have tried them both.