So what is the purpose of the upgrade? I can't imagine you needing more power to drive the Sonatina II's, they are fairly sensitive at 93db. Do you prefer the sound of the 7B-ST or 14B-ST? The 7B-ST operates in high-current or high-voltage mode (series or parallel, I forget which is which). High-current mode isn't recommended for speakers over 3 ohms. The high-voltage mode is essentially the same as bridging a 4B-ST. I had the 7B-ST in my system for a one week audition. The biggest enhancement over the 4B-ST is stereo separation. There was an increase in dynamics in high-current mode and a slight sweetness in the treble but you couldn't use it with your speakers.
If you have money to spend and want to improve your system I would consider power conditioning, power cords for your source, IC's, or speaker cables.
If you have money to spend and want to improve your system I would consider power conditioning, power cords for your source, IC's, or speaker cables.