Brigid Monos or Mono Blocks?

A bridged amp sees half the load impedance. This can be a problem with certain speakers and amps. Current demand doubles. 

One advantage of bridging capability:

say switch to tubes, keep cost/heat/size/weight down, will 45 wpc be enough?

or: buy new speakers, less sensitivity, existing amp enough power for them?

If reasonably easy to find another, used or new, you can bridge them later, typically getting double the power.

btw: two sets of outputs from the preamp? 

When I’ve tried stereo/mono bridge-able Solid State, the bridging brings a lot more power - up to 4x power, if the amp truly "doubles down" because its PSU / output stage / heat sinks can handle the strain. But more often you’ll actually see a ~ 3x power gain or thereabouts. HOWEVER this approach is not without sonic downsides. It’s turned me off of bridging. Not only are you running the amp much harder, but the bit of bridging circuitry itself may be a less than ideal implementation. And yes each channel now "sees" only half of your speaker’s impedance - so if your speakers are nominal 4 ohms, your amp would need to safely drive 2 ohms (lower for dips, so it should be stable to 1 ohm).

The switchable stereo/mono tube amps I’ve had (VAC) use a different approach than bridging. They parallelize the 2 stereo channels at the output transformers. This approach "only" doubles output power, BUT the sonic impact is ALL positive - no downsides like with bridging. This approach kind of has the opposite impedance problem of bridging. The output "taps" will shift around. What was formerly an 8 ohm optimized tap (stereo mode) is now optimized for 4 ohms, due to the parallelization. So you'd need 16 ohms stereo taps to run "optimal" mono into 8 ohms. VAC doesn't supply those 16 ohms taps, and my speakers are 8 ohms. But it still works great :)

@mulveling 

So am I interpreting your response correctly in that it would be better to use mono blocks rather than bridge two Stereo amps? SimAudio has a unusual way of  Bridging the 330A Stereo amp. They sell a XLR Y-daptor cable that on one end is two XLR connectors. One for each input channel and a single XLR on the other end to accept your source.