The Reference 3A Decapo’s are phased aligned speakers and in answer to your question then as well as now no, a properly integrated sub can not only add the lower frequencies but flesh out the mid bass through the upper frequency range. One of my earliest experiencies with the benefits of a properly integrated subwoofer was with Vandeersteen 2C speakers in a properly set-up and treated room over 30 years ago, subs, there were 2 subs in that setup both Vandersteen as well. With my current setup there in absolutely no loss of anything least of all tone. The amp is working less, lower distortion, less noise/artifacts, thus greater clarity, detail, dynamics and tone. The room is treated and the subs were adjusted using a spectrum analyzer. One thing I would add two subs offer a significant improvement over one.
I would speak with John the of Audio Connection in New Jersey. I’m sure he could help you with any questions regarding your 1ci’s. Good Luck Jim!