The Aspen letter follows a letter from Endurance sent to Aspen shareholders earlier that same day criticizing Aspen's standalone plan.

“Don't be fooled by Aspen's dubious assurances about its 'standalone plan,'” the Endurance letter, signed by Chairman and CEO John Charman, states. “Under the stewardship of its current board and management, Aspen's performance has lagged that of Endurance across key metrics, including underwriting profitability (i.e., combined ratio), diluted book value per share growth and share price performance. Despite the efforts of Aspen's board and management to distort the truth and confuse shareholders, there is no denying the facts….”

The letter adds charts showing side-by-side performance metrics based on company results.

Aspen's letter, though, disputes Endurance's contentions, stating, “In a letter filed publicly this morning, Endurance made a number of erroneous and ill-informed claims about Aspen's business, which underscores our deep concern about their failure to understand the significant dis-synergies that would result from the misguided transaction they are proposing.”

It includes side-by-side comparisons of its own on certain metrics. For example, Endurance's letter shows a favorable combined ratio in comparison to Aspen, while Aspen's letter contends its accident-year combined ratio, excluding the impact of reserve releases, compares favorably to Endurance.

As for its Q2 preliminary results, Aspen says it expects:

• Diluted book value per share of between $44.60 and $44.80; up 4.4% to 4.9% from the end of Q1.
• Diluted operating earnings per share between $1.30 and $1.35.
• Diluted earnings per share between $1.70 and $1.75.
• Gross written premiums between $775 and $780 million.
• Combined ratio between 90 and 91 or 89 to 90, excluding bid defense costs.
• Net favorable reserve development equating to between 4.5 and 5.5 combined-ratio points.
• Annualized operating return on equity between 12% and 12.8%
• Annualized net income ROE between 16% and 16.8%

A UBS analysis of Aspen's preliminary report comes with the title, “The Best Defense is Good Earnings,” and notes the expected operating ROE of 12% to 12.8% exceeds Aspen's target of 10% for the full year of 2014.

The UBS analysis says the improved ROE and higher book value per share may pressure Endurance to again raise its offer.

UBS says, “Strategically, we continue to believe a combination of [Aspen] and [Endurance] makes sense given the benefits of a larger balance sheet (particularly in the increasingly competitive reinsurance business), and the capital and expense efficiencies that would likely be generated. We continue to believe that a fair takeout value for AHL is in the range of 1.2x – 1.3x fully diluted BVPS, or $53 – $58 per share taking into account [Q2] results.”