A dedicated preamp would definitely be an upgrade, however I think the amp/speaker match is just OK. An 8 ohm speaker that dips to 3.8 ohms is not the best for a tube amp. Also, the 706v has been described as having bass power. If a speaker and amp don't match well, I think the bass suffers the most because it is the most demanding.
What does the system sound like using the 640a only with its power? Soundstage, highs, lows, dynamics?
What does the system sound like using the 640a only with its power? Soundstage, highs, lows, dynamics?