One thing to consider, The Mark V has been around since 2009 and there may have been some tweaks over the years (assumed). Not sure if the term "Bloom" is appropriate here but there are some differences between the other Mesa amps regarding the reverb. I can hear a stepped wash or bloom on the reverb of the Roadster (depending on set level per channel). This is similar to the Mark V but seems more subtle than dramatic. JP-2C has more of an analog bloom such that I do not hear any level shifting (gives it a wash sound) or sudden cut-out like the Mark V. Switching from a high gain setting to a low or clean channel has its complexities as each channel is sharing the same tubes and they may be operating in different regions and plate voltages. Sudden level shits cause by relays switching out parts of the circuits and activating others will generate electrical noise that will be audible and having the noise transfer into the reverb circuit would sound far worse than the delay. During the channel switch, the entire signal path is muted before the switch takes place and then release after the circuits stabilize with the change in operating characteristic. However due to where the reverb is fed, the level of the new signal may be stronger than the previous level that feeds the reverb send. If the bloom is too slow it will be noticed and if the effect is taking longer than desired it will not sound good and become a distraction. I would first experiment with different reverb levels for each channel. If you are using tweed on the clean channel the reverb is essentially reduced no matter what the control setting is for the reverb level on CH1. Since CH2 and CH3 operate the reverb circuit at a reduced signal strength, changing to the clean channel with a rich reverb setting will definitely be noticed.
External reverb comment: I would agree that there are some phenomenal reverb pedals on the market these days. One of my favorites is the Strymon Big Sky but they also make a smaller version at a lower price which is also a good pedal (blue sky). Note: also works excellent in the Mark V loop without loosing any integrity of the signal. Also you have the ability to blend the dry and wet signal to taste so it is not overbearing. If you plan on going the external reverb route, make sure the pedal can operate at line level as well as instrument level.
External reverb comment: I would agree that there are some phenomenal reverb pedals on the market these days. One of my favorites is the Strymon Big Sky but they also make a smaller version at a lower price which is also a good pedal (blue sky). Note: also works excellent in the Mark V loop without loosing any integrity of the signal. Also you have the ability to blend the dry and wet signal to taste so it is not overbearing. If you plan on going the external reverb route, make sure the pedal can operate at line level as well as instrument level.