Brief Answer:
Follow the recommendations listed below
Detailed Answer:
Hi Mr. XXXXXXX 
Thanks for posting your query on this forum.
*     Cramps are common in Predominant Constipation type of IBS. Relief from constipation will reduce the cramps.
*     
Tamsulosin is not known to have any adverse effect on IBS. So, you may continue with it for 
enlarged Prostate.
Further, keeping your primary concerns in mind, I hereby recommend the following course of action :-
#    In addition to Verapamil, take a long-term (3-12 months)regular course of a combination of an anti-depressant (such as, 
Citalopram)  + anti-anxiety drug (such as, Alprazolam or 
Clonazepam), which will drastically cut down the symptoms of IBS.
#    3-4 weeks' course of a 
Probiotic, such as, Lactobacilli; and, pancreatic digestive enzymes.
#    For 
chronic constipation, the safest and equally remedy is to take Psyllium husk TWICE DAILY for an adequate duration, as per your treating doctor’s advice. The dose may be reduced once the bowel movements normalize.
#   Though, cramps will disappear following correction of constipation, an anti-spasmodic drug, such as, Tab 
Dicyclomine (20mg) may be taken as and when required.
#   Other Precautions/measures :-
 
    -     eat slowly with adequate chewing of food; 
    -     high fiber diet and plenty of water intake; 
    -     inclusion of stool bulking agents, such as XXXXXXX in diet;
    -     Do not chew gum or take carbonated/aerated drinks; 
    -     reduce excess weight;
    -     Regular exercises.
I am confident that, with the above medication and precautions, your IBS will remain in remission.
Consult your doctor and apprise him of my opinion. I am certain that he will agree  with me and  prescribe suitable treatment as advised.
Hope I have addressed your concerns to your complete satisfaction and you find my response helpful and informative. I will be happy to answer further queries, if any.
Fond regards and have a nice day,
Col (Dr.) Rakesh Karanwal