I had a Dyna 400 that was a factory build, roughly 1973 as I recall. I got an accommodation since I worked part time in an audio store. I liked the idea of the "Dynaguard" (running from memory here), it performed well. I swapped it out for an original Phase Linear 700 which was a beast and yes, it did flame out, but got fixed promptly by Carver back in the day. (We are talking 1973 still). I used stacked Advents and went through my share of tweeters. Then did an about face, bought a pair of Quad Loudspeakers, built a Stereo 70 as a stop gap and within a few years, ARC preamp and amp. I've been in tube world since then.
I have no idea what it is worth today. Solid state stuff back then was not a smooth, it had a more pronounced "grain" in the mids- not specifically the Dynaco 400 but I remember the Phase Linear 400 sounding better than the 700 at the time. Whether it is worth a recap-- probably depends on how attached you are to it.
I still have my ARC Dual 75a-- which I kept for sentimental reasons. I recapped it in 2003 or so through an authorized facility and it was used intermittently as a rear channel home theater amp with two other ARC tube amps. I eventually got tired of the heat generated by all those tubes running in a home theater application and switched to a big McI solid state multi-channel basic power amp, which I still have.
I don't know how many Dyna 400s are around or how that cuts in terms of market value should you want to sell it. We've come a long way in solid state since then though I'm still a tube devotee.