300b vs 845

If the power supply design is correct and of adequate capacity heat should not be an issue with a correctly designed 300b. My Audio Note Conqueror was a space heater; my custom 300b tube rectified amp runs warm but does not materially impact the ambient temperature.

Assuming the amps are of similar quality you will have more power with the 845. All things being equal expect the 845 to be more expensive simply due to the larger chassis and transformers required. The more power you get the more heat the amp will generate; be warned if you live in a hot area and do not have AC.

The ASL stuff is mid fi and average parts and transformers are used. You want to look at quality 300b/845 amps look at Art Audio, Border Patrol, DeHavilland, or custom. You get what you pay for is always true.