I’ll take a stab at a few of your questions:
2. I use Silent Running Audio VR series isolation platforms. It’s a bit of an up front investment, but they are effective, easy to use and look nice.
3. I don’t use power conditioners, but do have dedicated 20 amp lines run to my audio system.
4. I use digital exclusively (no vinyl). Many opinions out there and you can spend as little as $1,500 on an entry level streamer paired with an entry level DAC. I’ve used Bluos with an external DAC, which is a good dependable introduction to streaming. Also tried an Aurender N10, but it was very glitchy, even after sending it back to the US Service Center, so I’d stay away from Aurender (but many seem to love their products). Innuos seems to get a lot of love, with few, if any detractors... so I’d give them serious consideration and pair it with a DAC of your choosing. I use a ROON Nucleus paired with an IFi Zen Stream ROON endpoint/streamer and a Weiss 202 DAC... I love it, but I don’t recommend it for a newbie, unless you are technically inclined or have patience with electronics/IT stuff. You may want to go to a reputable dealer that has return privileges, as digital can get tricky and some systems might not play nice with your cable/wireless provider.
5. I have Altec Model 19’s and find that as long as the amp pairs well with them, you should not need a sub as Model 19’s have excellent bass, assuming your amp is a good fit. I’ve found out the hard way that despite the 19’s high efficiency rating, they really benefit from more power than you’d think. I tried some low power 300B SET amps, none of which had enough juice to put out adequate bass or dynamics. The best amps I’ve used with it are higher powered tube amps. If you are unable to get strong bass and dynamics from the MC 240, you’ll either need a more powerful amp (> 75 wpc), or go back to Proac’s or maybe Splendor’s. I’ve never been successful in implementing subs with previous speakers and I’m just not a fan. Trust that the 19’s have ample bass, slam and dynamics when paired with the right amplifier. They are special speakers, but can take a bit of work with regards to tweaking upstream gear to maximize their potential. Feel free to message me privately if you want to discuss 19’s, as I’ve refurbished my drivers and added new crossovers and have tried at least 7 amps with them.