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.

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.

In "most" cases, yes that would be my assertion for solid state amps (tube amps are generally fine either way): you’re better off with dedicated mono amps than a pair of bridgeable stereo/mono amps. However that is always subject to the specific amp in question.

If SimAudio does what you say, it sounds like they’re bypassing the need for any special "bridging" input circuitry by utilizing the + and - phases already present in a balance source signal (L channel gets + and R gets - or vice versa). That might work really well, but you’ll run into trouble if your source signal isn’t truly balanced (e.g. if you used RCA - to - XLR adapters). And you’ll still be driving the amps harder, so at that point in depends on how well the amp does when seeing half your speaker’s impedance. If it can handle that without too much added distortion, then the positives might outweigh the negatives by enough to make the extra expense worthwhile.