Not really, at least if you are playing normally. You won't be affording Dread knight for your first 5-6 ascensions, minimum.
Due to how things scale, each boss has roughly double the health of the boss before it. so you will hit walls in your progress that are effectively impassable by simply grinding for gold, 140 is the first one. You need to ascend and get the bonuses from Hero Souls and Ancients to break through those walls.
It's not really mentioned anywhere but, each hero soul adds a 10% DPS bonus, yes?
and do you earn these souls from leveling your heroes, advancing levels or killing monsters?
I noticed some boss fights reward them...
You earn 1 soul for every 2,000 levels in total between your heroes. Early on this will likely be at least half of the souls you earn per ascension. You will usually want to ascend with wither 14,000 or 16,000 hero levels for your first few ascensions.
The Centurion bosses every 100 zones will always drop souls based on the zone level. Other bosses past zone 100 have a base 25% chance to be Primals which will drop souls the first time they are defeated on a particular run. Bosses will stay Primal if you have to drop back a level to farm gold, but will be reset to random after you ascend. The number of Souls dropped by a boss depends on the zone it spawns in and it generally increases by one for every 15 zones further on. The accurate formula is HS=((((Zone-80)/25)^1.3)*(1+Solomon bonus)).
Note that you will actually obtain the souls after you ascend, the souls you earn during a particular run won't actually effect that run itself.
Also note that the Quick Ascension is the best value per ruby of the items that you can buy with the rubies that drop, but hold off on using it until you are at the point where you would normally ascend. The souls given by a QA are the average number of souls that would be dropped by bosses in a run that reaches the highest zone you have ever cleared + 7 extra souls that take the place of the souls you gain from hero levels. These go straight into your bank and so will effect your damage for the remainder of your current run, which will normally allow you to push a number of zones further.
Read through the
Beginners guide I wrote, most of the basics are explained there.