I have the 604 D and have found tubes really sweet with these speakers. While not knowing a SS amp to recommend, I would suggest a nice integrated, maybe like the sli-80. Don't need much power, you could go all the way down to a 7 watt set and still drive the 604 very nicely.
A decent preamp for Altec 604 speakers
I am addicted to high efficiency speakers. I own Altec 620 monitors (6048G). Outstanding but require very good electronic. The mids tend to magnify poor elements ... Some told me to get a tube preamp... I had a few tube amps in the past and got sick of all the care they need. So I rather go with SS. I need transparency and a very smooth medium. I listen to vinyl, mostly acoustic and electric pop. Up to 1k for a used gear. Thanx.