Welcome to the forum! Nice to see another Dyncao builder hanging around!
First thing's first: It would be good to know how large of a listening room you are in and how loud you typically listen to music. That can help to make recommendations.
I have built a Dynaco ST-70 from the Bob Latino (Tubes 4 HiFi) kit. Running EL34 output tubes.
To date, I have run my ST-70 on the Infinity P363 towers, the Philharmonitors, and the Def Tech SM350s. All in a relatively small room.
Here are my opinions:
*Philharmonitors: Don't bother. At low levels it was fine. But at slightly higher listening levels, I could hear the ST-70 struggling. This may be in part to lesser sensitivity from the Phils, but probably as much due to the un-forgiving transparency of the Phils.
*P363: I ended up going back into the ST-70 to install the 4-ohm taps. This greatly improved the sound and power delivered of the P363 as compared to the 8-ohm taps. No surprise, as the P363 was determined to be 4-ohm as measured here on AH.
*Def Tech SM350: Dang, the ST-70 really shines on these speakers! When properly placed, these speakers have more bass than you would expect for a smaller bookshelf. I'm sure these speakers are more forgiving than many, so that helps on the ST-70. I bought a pair from NewEgg for $200 a while back, and I say these are dang tough to beat if you can get that price!
So, from my own personal experience, I would suggest a pair of DT SM350, or even better spring for the SM450. The 350 can be found for $200 a pair if you are patient, and I have seen the 450 as low as the $300 a pair range. At those prices, they are good speakers, at full retail there are better options.
No personal experience, but a few of the AH members have suggested that a pair of vintage Klipsch Heresy speakers would also be a good match for the ST-70.
Now, to be clear, to get the BEST performance out of your ST70: I recommend getting a pre-amp or receiver with bass management and adding a subwoofer to the system. That will get you around one of the biggest limitations of the ST70 and / or Bookshelf speakers.
I had been running my ST-70 permanently on my P363s, being fed a source from drum machines and synths, but 1 of my channels on my ST-70 recently quit. I just haven't had a chance to really deal with troubleshooting that.