When you say two lower gears, you presumably have 25 and 27 as the largest gears in your cassette. Does that mean 39/31 would be low enough? If so, the compact with a 34 will get you there, but 34/25 will be too high. Shimano makes a 12/30 10speed cassette in the CS4600 line, which I have coupled to a 50/34 compact. You will be in for a new rear derailleur that way. They also make a 11/32 11speed, but that needs new shifter as well. This might avoid the crankset change, however.
Ross Millikan
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