You are correct, the capacitors were/are probably the least expensive that met specifications. They do not look to be bulging in the picture. If you were to remove them and measure their ESR's, you might find some are increased. Nevertheless, if you are set on replacing them, I suggest just using general purpose, high quality ones with a high ripple voltage rating. That is, given two of similar ratings, pick the one of the same size with a higher ripple rating.
There is also a temperature rating, often 85°C vs. 105°C and a lifteime rating, often something like 1000 hours to 2000 hours or more. If you are just getting a few, go for the higher temperature and lifetime ratings. There really are a lot of specifications to chose from, and what you want is something that works. A dollar or two difference in cost won't make much difference for a one off compared to the difference it makes for a million.
John